Ultrasonic densitometer with pre-inflated fluid coupling membranes

ABSTRACT

A contained water ultrasonic densitometer employs pre-inflated bladders holding coupling fluid about ultrasonic transducers. The bladders in a relaxed state define a cavity smaller than the human heel so that when the heel is inserted, the bladders conform to the heel in a wiping action removing entrapped air.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 09/277,481filed Mar. 26, 1999 which claims the benefit of provisional applicationNo. 60/080,158 filed Mar. 31, 1998 and is a continuation-in-part of U.S.patent application Ser. No. 09/094,073 filed Jun. 9, 1998 now U.S. Pat.No. 6,027,449, which is a continuation-in-part of U.S. patentapplication Ser. No. 08/795,025 filed Feb. 4, 1997, now U.S. Pat. No.5,840,029 which is a continuation-in-part of 08/466,495 filed Jun. 6,1995, now U.S. Pat. No. 5,603,325, which is a continuation-in-part ofU.S. patent application Ser. No. 08/397,027 filed Mar. 1, 1995, now U.S.Pat. No. 5,483,965 which is a continuation of U.S. patent applicationSer. No. 08/072,799 filed Jun. 4, 1993, abandoned, which is acontinuation-in-part of U.S. patent application Ser. No. 07/895,494filed Jun. 8, 1992, now U.S. Pat. No. 5,343,863 which is acontinuation-in-part of U.S. patent application Ser. No. 07/772,982filed Oct. 7, 1991, now U.S. Pat. No. 5,119,820, which is a continuationof U.S. patent application Ser. No. 07/343,170 filed Apr. 25, 1989 nowU.S. Pat. No. 5,054,490, which is a continuation-in-part of Ser. No.07/193,295 filed May 11, 1988, now U.S. Pat. No. 4,930,511.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ultrasonic densitometer equipment usingultrasonic sound waves to measure bone integrity, and in particular, toan ultrasonic densitometer in which the ultrasound is conducted to ahuman heel through liquid filled bladders.

2. Description of the Prior Art

Various devices presently exist which may be used to measure thephysical properties and integrity of a member such as a bone.Non-invasive density measuring devices can be used to determinecumulative internal damage caused by micro-crushing and micro-fracturingoccurring in the bones of humans or animals such as race horses.Additionally, osteoporosis, or loss of bone mineralization, detection inhumans and its cure or prevention are increasingly becoming areas ofintense medical and biological interest. As the average age of the humanpopulation increases, a greater number of patients are developingcomplications due to rapid trabecular bone loss.

Early Work

U.S. Pat. No. 3,847,141 to Hoop discloses a device for measuring thedensity of a bone structure, such as a finger bone or heel bone, tomonitor the calcium content thereof. The device includes a pair ofopposed spaced ultrasonic transducers which are held within a clampingdevice clamped on the bone being analyzed. A pulse generator is coupledto one of the transducers to generate an ultrasonic sound wave which isdirected through the bone to the other transducer. An electric circuitcouples the signals from the receive transducer back to the pulsegenerator for retriggering the pulse generator in response to thosesignals. The pulses therefore are produced at a frequency proportionalto the transit time that the ultrasonic wave takes to travel through thebone structure which is directly proportional to the speed of the soundthrough the bone. The speed of sound through a bone has been found to beproportional to the density of the bone. Thus the frequency at which thepulse generator is retriggered is proportional to the density of thebone.

Another device and method for establishing, in vivo the strength of abone is disclosed in U.S. Pat. Nos. 4,361,154 and 4,421,119 to Pratt,Jr. The device includes a launching transducer and a receivingtransducer which are connected by a graduated vernier and whichdetermine the speed of sound through the bone to determine its strength.The vernier is used to measure the total transit distance between thesurfaces of the two transducers.

Lees (Lees, S. (1986) Sonic Properties of Mineralized Tissue, TissueCharacterization With Ultrasound, CRC publication 2, pp. 207-226)discusses various studies involving attenuation and speed of soundmeasurements in both cortical and spongy (cancellous or trabecular)bone. The results of these studies reveal a linear relationship betweenthe wet sonic velocity and wet cortical density, and between the drysonic velocity and the dry cortical density. The transit times of anacoustic signal through a bone member therefore are proportional to thebone density. Langton, et al. (Langton, C. M., Palmer, S. D., andPorter, S. W., (1984), The Measurement of Broad Band UltrasonicAttenuation in Cancellous Bone, Eng. Med., 13, 89-91) published theresults of a study of ultrasonic attenuation versus frequency in the oscalcis (heel bone) that is utilized through transmission techniques.These authors suggested that attenuation differences observed indifferent subjects were due to changes in the mineral content of the oscalcis. They also suggested that low frequency ultrasonic attenuationmay be a parameter useful in the diagnosis of osteoporosis or as apredictor of possible fracture risk.

Current Devices

A common site for the ultrasonic measurement is the os calcis of thehuman heel. In one ultrasonic densitometer design for measuring thissite, opposed ultrasonic transducers are placed on opposite sides of areceptacle sized to hold the human foot. The receptacle is filled withwater of a controlled temperature which serves to couple the ultrasonicenergy from a first transducer, through the gap between that transducerand the human heel, and from an exit point of the human heel backthrough a corresponding gap between the exit point and the receivingtransducer. To the extent that the water approximates the acousticimpedance of the soft tissue surrounding the heel, coupling ofultrasonic energy through the heel is improved and the precision of themeasurement increased.

While a water filled receptacle provides a simple mechanism for couplingultrasonic energy to the heel, it may be desired to contain the waterbehind a flexible membrane or the like so as to reduce the possibilityof spilling or contaminating the water. It is known to contain waterwithin flexible bladders and to mount those bladders on an adjustingmechanism so that the bladders may be moved to compress the heel betweenthem. This movement of the bladder supports may be augmented withlimited inflation of the bladders and/or movement of the transducers.Such compressive type bladder systems create a risk of air entrapmentbetween the bladders and the heel such as may affect the quality of themeasurements. For this reason it is known to use a conical shapedmembrane that initially contacts the heel at a single point and, withfurther compression, expands outward to reduce air entrapment. Suchmembranes are difficult to fabricate and do not conform well toessentially flat opposed surfaces of the heel.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a contained-water ultrasonic densitometerusing stationary pre-inflated bladders. The bladders are arranged toform a cavity between them slightly smaller than the heel and the heelis slid in between the bladders. The sliding provides a wiping actionthat helps eliminate air trapped between the bladders and the foot.

Specifically, the densitometer provides pre-inflated bladders opposedalong an ultrasonic propagation axis. A coupling material is containedwithin the bladders and ultrasonic transducers are positioned within thebladders to direct ultrasonic signals through the coupling materialalong an ultrasonic propagation axis between the transducers. Thebladders are compliant so as to permit them to move apart to allowinsertion of a human heel. The shape and composition of the bladdersurfaces allow the bladders to slide over and conform with the heelwhile remaining substantially in alignment with the propagation axis.

Thus, it is one object of the invention to provide a contained-waterultrasonic densitometer having significantly decreased air entrapmentbetween the bladders and the foot. The sliding action of the footagainst the compliant membranes tends to reduce and eliminate entrappedair.

The first ultrasonic transducer may be held in opposition to a secondultrasonic transducer both with fixed separation and the first andsecond bladder surfaces may be mounted to a fixed support.

Thus it is another object of the invention to provide a greatlysimplified mechanism for a contained water ultrasonic densitometereliminating the need for a sliding mechanism to engage the bladders withthe foot or repeated pumping and deflation of the bladders betweenmeasurements.

The bladder surfaces may be comprised of an elastomeric membrane havinga surface coding of ultrasonic coupling gel.

Thus it is another object of the invention to provide a surface thateasily slides along the surface of the human heel while also providinggood coupling for the last interface between the bladder surface and thehuman heel.

The foregoing and other objects and advantages of the invention willappear from the following description. In this description, reference ismade to the accompanying drawings which form a part hereof and in whichthere is shown by way of illustration, a preferred embodiment of theinvention. Such embodiment does not necessarily represent the full scopeof the invention, however, and reference must be made therefore to theclaims for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of the ultrasound densitometer deviceconstructed in accordance with the present invention;

FIG. 2 is a perspective view of an acoustic coupler, two of which areshown in FIG. 1;

FIG. 3 is a front view of a transducer face from which acoustic signalsare transmitted or by which acoustic signals are received, the face ofthe other transducer being the mirror image thereof;

FIG. 4 is a schematic block diagram view of the circuitry of theultrasound densitometer device constructed in accordance with thepresent invention;

FIG. 5 illustrates the method of sampling a received waveform used bythe circuit of FIG. 4;

FIG. 6 is a schematic block diagram view of the circuitry of analternative embodiment of an ultrasound densitometer constructed inaccordance with the present invention;

FIG. 7 is a sample of an actual ultrasonic pulse and response from anultrasonic densitometer according to the present invention;

FIG. 8 is a sample plot of relative ultrasound pulse intensity overfrequency range;

FIG. 9 is a graph in frequency domain illustrating the shift inattenuation versus frequency characteristic of a measured object ascompared to a reference;

FIG. 10 is a perspective view of an alternative embodiment of thepresent invention showing a basin for receiving a patient's foot andhaving integral opposed ultrasonic transducers;

FIG. 11 is a plan view of a foot plate and toe peg used with theembodiment of FIG. 10;

FIG. 12 is a cross-sectional detail of the foot plate of FIG. 11 showingthe method of attaching the sliding toe peg of the foot plate;

FIG. 13 is a block diagram of a system for transporting the acousticcoupling liquid used in the embodiment of FIG. 10;

FIG. 14 is a schematic block diagram view of the circuitry of theembodiment of FIG. 10;

FIG. 15 is an exploded view of the underside of the foot basin of FIG.10 showing a c-clamp for holding the opposed ultrasonic transducers inprecise alignment and separation;

FIG. 16 is a perspective detailed view of the shank of the c-clamp ofFIG. 15 showing a lever for moving the separation of the transducersbetween an open and precisely separated closed position;

FIG. 17 is a cross-section of a human heel and ultrasonic transducers ofthe basin of FIG. 10 showing flexible liquid filled bladders surroundingthe transducers and providing a coupling path between the transducersand the heel;

FIG. 18 is a plot of the inverse of time of flight (TOF) for two boneconditions and broadband ultrasonic attenuation (BUA) as a function ofheel width showing their opposite functional dependencies;

FIG. 19 is a plot of bone quality versus bone width as might be obtainedfrom empirical measurement of multiple bone phantoms and as may be usedto eliminate bone width effects in the ultrasonic assessment of bonequality;

FIG. 20 is an exploded view of the elements of an ultrasonic detectorarray showing a driving mechanism for improving the resolution of theacquired data and the location of a piezoelectric film detector arrayabove a spatially offset connector;

FIG. 21 is a detailed perspective fragmentary view of the piezoelectricfilm detector with electrodes on its surface as communicating withconnector terminals via acoustically transparent conductors;

FIG. 22 is a detailed fragmentary view of the piezoelectric film showinga method of assembling acoustically transparent conductors;

FIG. 23 is a detailed view of the face of the detector showing itsdisplacement by the driving mechanism of FIG. 20;

FIG. 24 is a figure similar to that of FIG. 17 showing use of thedetector array to provide focused reception at a point within apatient's heel;

FIG. 25 is a perspective view in phantom of a patient's heel showing araster scan pattern of a reception point within the heel to measurevolumetric bone density variations within a inner and outer portion ofthe os calcis;

FIG. 26 is a schematic representation of a data cube collected in thescanning shown in FIG. 25 with isodensity lines used to locate ameasurement region of interest;

FIG. 27 is a flow chart of the operation of the present invention inlocating a region of interest uniformly over several patient visits;

FIG. 28 is a perspective view of an embodiment of the invention using afixed focus transducer array mechanically scanned to provide a pluralityof spatially separated measurements;

FIG. 29 is a partial cross-sectional view of an alternative embodimentto FIG. 17 using overlapping inflatable bladders;

FIG. 30 is a figure similar to FIG. 17 showing the bladders inflated ina first mode using a first coupling liquid;

FIG. 31 is a figure similar to that of FIG. 30 showing the bladdersinflated in a second mode using a second coupling liquid in the innerbladder.

FIG. 32 is a perspective exploded view and partial cutaway of an outerbladder element adapted for disposable use;

FIG. 33 is a flowchart describing a program executed with amicroprocessor 38 of FIG. 4 using the bladder configuration of FIG. 29;and

FIG. 34 is a graph representing variations in value of attenuation,speed of sound and a combined stiffness quantity as a function of time.

FIG. 35 is a simplified cross-sectional view of a flexible membraneaccording to the present invention showing a hemispherical distentionfor improved patient immobilization;

FIG. 36 is geometric diagram showing representational stretching ofmembranes with cross axis patient movement for membranes of twodifferent aspects;

FIG. 37 is a cross sectional view of a closed system for distending twomembranes using a rolling diaphragm pump element;

FIG. 38 is a perspective view of a contained-water ultrasonicdensitometer system of the present invention showing a receptacle forreceiving a human foot holding opposed pre-inflated bladder surfaces;

FIG. 39 is a cross-sectional view along line 39—39 of FIG. 38 showingconfiguration of the bladders within the receptacle prior to insertionof a human foot;

FIG. 40 is a figure similar to that of FIG. 39 showing deformation ofthe bladders and engagement of the heel against a stop plate uponinsertion of the heel within the receptacle of the ultrasonicdensitometer;

FIG. 41 is a cross-sectional view taken along lines 41—41 of FIG. 38showing seating of the foot against the stop plate of FIG. 40 andmeasurement of the forces so produced to ensure proper seating of thefoot; and

FIG. 42 is a chart showing downward and backward forces imparted by thefoot on the stop plate and a defined region of proper force enabling ameasurement by the densitometer.

DETAILED DESCRIPTION OF THE INVENTION Caliper Embodiment

Referring more particularly to the drawings, wherein like numbers referto like parts, FIG. 1 shows a portable ultrasound densitometer 10 formeasuring the physical properties and integrity of a member, such as abone, in vivo. The densitometer 10 as shown in FIG. 1 includes a handle11 with actuator button 12. Extending linearly from the handle 11 is aconnection rod 13. The densitometer 10 also includes a fixed arm 15 andan adjustable arm 16. The fixed arm 15 preferably is formed continuouslywith the connection rod 13, and therefore is connected to an end 17 ofthe connection rod 13. The adjustable arm 16 is slidably mounted on theconnection rod 13 between the handle 11 and a digital display 18 mountedon the rod 13. The knob 19 may be turned so as to be locked or unlockedto allow the adjustable arm 16 to be slid along the connection rod 13 sothat the distance between the arms 15 and 16 may be adjusted.

Connected at the end of the fixed arm 15 is a first (left) transducer 21and at the end of the adjustable arm 16 is a second (right) transducer21. As shown in FIGS. 1 and 2, each of the transducers 21 has mounted onit a respective compliant acoustic coupler 23 to acoustically couple thetransducer to the object being tested. The acoustic coupler 23 includesa plastic ring 24 and attached pad 26 formed of urethane or othercompliant material. FIG. 3 shows a face 28 of the first (left)transducer 21 which is normally hidden behind the compliant pad 26 ofthe acoustic coupler 23. The transducer face 28 normally abuts againstthe inner surface 29 of the pad 26 shown in FIG. 2. The transducer face28 shown in FIG. 3 includes an array of twelve transducer elementslabeled a-l. The second (right) transducer 21 includes a face 28 whichis the mirror image of that shown in FIG. 3.

FIG. 4 generally shows in schematic fashion the electronic circuitry 31of the densitometer 10, which is physically contained in the housing ofthe digital display 18. An object 32 is placed between the twotransducers 21 so that acoustic signals may be transmitted through theobject. This object 32 represents a member, such as a bone, or somematerial with known acoustic properties such as distilled water or aneoprene reference block. As shown in the embodiment illustrated in FIG.4, the leftmost transducer 21 is a transmit transducer and the rightmosttransducer 21 a receive transducer. In fact though, either or both ofthe transducers 21 may be a transmit and/or receive transducer. Thetransmit and receive transducers 21 of the circuit of FIG. 4 areconnected by element select signals 36 and 37 to a microprocessor 38.The microprocessor 38 is programmed to determine which one of therespective pairs of transducer elements a through l are to betransmitting and receiving at any one time. This selection isaccomplished by the element select signal lines 36 and 37, which may beeither multiple signal lines or a serial data line to transmit theneeded selection data to the transducers 21. The microprocessor 38 isalso connected by a data and address bus 40 to the digital display 18, adigital signal processor 41, a sampling analog to digital converter 42,and a set of external timers 43. The microprocessor 38 has “on board”electrically programmable non-volatile random access memory (NVRAM) and,perhaps as well, conventional RAM memory, and controls the operations ofthe densitometer 10. The digital signal processor 41 has “on board”read-only memory (ROM) and performs many of the mathematical functionscarried out by the densitometer 10 under the control of themicroprocessor 38. The digital signal processor 41 specifically includesthe capability to perform discrete Fourier transforms, as iscommercially available in integrated circuit form presently, so as to beable to convert received waveform signals from the time domain to thefrequency domain. The microprocessor 38 and digital signal processor 41are interconnected also by the control signals 45 and 46 so that themicroprocessor 38 can maintain control over the operations of thedigital signal processor 41 and receive status information back.Together the microprocessor 38 and the digital signal processor 41control the electrical circuit 31 so that the densitometer 10 can carryout its operations, which will be discussed below. An auditory feedbackmechanism 48, such as an audio speaker, can be connected to themicroprocessor 38 through an output signal 49.

The external timer 43 provides a series of clock signals 51 and 52 tothe A/D converter 42 to provide time information to the A/D converter 42so that it will sample at timed intervals electrical signals which itreceives ultimately from the transmit transducer, in accordance with theprogram in the microprocessor 38 and the digital signal processor 41.The external timer 43 also creates a clock signal 53 connected to anexcitation amplifier 55 with digitally controllable gain. Timed pulsesare generated by the timer 43 and sent through the signal line 53 to theamplifier 55 to be amplified and directed to the transmit transducer 21through the signal line 56. The transmit transducer 21 converts theamplified pulse into an acoustic signal which is transmitted through theobject or material 32 to be received by the receive transducer 21 whichconverts the acoustic signal back to an electrical signal. Theelectrical signal is directed through output signal 57 to a receiveramplifier 59 which amplifies the electrical signal.

The excitation amplifier circuit 55 is preferably a digitallycontrollable circuit designed to create a pulsed output. Theamplification of the pulse can be digitally controlled in steps from oneto ninety-nine. In this way, the pulse can be repetitively increased inamplitude under digital control until a received pulse of appropriateamplitude is received at the receiver/amplifier circuit 59, where thegain is also digitally adjustable.

Connected to the receiver amplifier circuit 59 and integral therewith isa digitally controllable automatic gain control circuit which optimizesthe sensitivity of the receive transducer 21 and the amplifier circuit59 to received acoustic signals. The microprocessor 38 is connected tothe amplifier circuit and automatic gain control 59 through signal line60 to regulate the amplification of the amplifier circuit and gaincontrol 59. The amplified electric signals are directed through lead 61to the A/D converter 42 which samples those signals at timed intervals.The A/D converter 42 therefore in effect samples the received acousticsignals. As a series of substantially identical acoustic signals arereceived by the receive transducer 21, the A/D converter 42progressively samples an incremental portion of each successive signalwaveform. The microprocessor 38 is programmed so that those portions arecombined to form a digital composite waveform which is nearly identicalto a single waveform. This digitized waveform may be displayed on thedigital display 18, or processed for numerical analysis by the digitalsignal processor 41.

The densitometer constructed in accordance with FIGS. 1-4 can beoperated in one or more of several distinct methods to measure thephysical properties of the member, such as integrity or density. Thedifferent methods, as described in further detail below, depend both onthe software programming the operation of the microprocessor 34 as wellas the instructions given to the clinician as to how to use thedensitometer. The different methods of use may all be programmed into asingle unit, in which case a user-selectable switch may be provided toselect the mode of operation, or a given densitometer could beconstructed to be dedicated to a single mode of use. In any event, forthe method of use of the densitometer to measure the physical propertiesof a member to be fully understood, it is first necessary to understandthe internal operation of the densitometer itself.

In any of its methods of use, the densitometer is intended to be placedat some point in the process on the member whose properties are beingmeasured. This is done by placing the transducers 21 on the oppositesides of the member. To accomplish this, the knob 19 is loosened toallow the adjustable arm 16 to be moved so that the transducers 21 canbe placed on opposite sides of the member, such as the heel of a humanpatient. The outside surfaces of the pads 26 can be placed against theheel of the subject with an ultrasound gel 35 or other coupling materialplaced between the pads 26 and subject 32 to allow for improvedtransmission of the acoustic signals between the member 32 andtransducers 21. Once the transducers 21 are properly placed on themember, the knob 19 may be tightened to hold the adjustable arm 16 inplace, with the transducers 21 in spaced relation to each other with themember 32 therebetween. The actuator button 12 may then be pressed sothat acoustic signals will be transmitted through the member 32 to bereceived by the receive transducer 21. The electronic circuit of FIG. 4receives the electrical signals from the receive transducer 21, andsamples and processes these signals to obtain information on thephysical properties and integrity of the member 32 in vivo. Themicroprocessor 38 is programmed to indicate on the digital display 18when this information gathering process is complete. Alternatively, theinformation may be displayed on the digital display 18 when theinformation gathering process is completed. For example, the transittime of the acoustic signals through the member 32 could simply bedisplayed on the digital display 18.

Considering in detail the operation of the circuitry of FIG. 4, thegeneral concept is that the circuitry is designed to create anultrasonic pulse which travels from transmit transducer 21 through thesubject 32 and is then received by the receive transducer 21. Thecircuitry is designed to both determine the transit time of the pulsethrough the member 32 (also called time of flight (“TOF”)), to ascertainthe attenuation of the pulse through the member 32, and to be able toreconstruct a digital representation of the waveform of the pulse afterit has passed through the member 32, so that it may be analyzed todetermine the attenuation at selected frequencies. To accomplish all ofthese objectives, the circuitry of FIG. 4 operates under the control ofthe microprocessor 38. The microprocessor 38 selectively selects,through the element select signal lines 36, a corresponding pair or agroup of the elements a through I on the face of each of the transducers21. The corresponding elements on each transducer are selectedsimultaneously while the remaining elements on the face of eachtransducer are inactive. With a given element, say for example element aselected, the microprocessor then causes the external timer 43 to emit apulse on signal line 53 to the excitation amplifier circuit 55. Theoutput of the excitation amplifier 55 travels along signal line 56 toelement a of the transmit transducer 21, which thereupon emits theultrasonic pulse. The corresponding element a on the receive transducer21 receives the pulse and presents its output on the signal line 57 tothe amplifier circuit 59. What is desired as an output of the A/Dconverter 42 is a digital representation of the analog waveform which isthe output of the single transducer element which has been selected.Unfortunately, “real time” sampling A/D converters which can operaterapidly enough to sample a waveform at ultrasonic frequencies arerelatively expensive. Therefore it is preferred that the A/D converter42 be an “equivalent time” sampling A/D converter. By “equivalent time”sampling, it is meant that the A/D converter 42 samples the output ofthe transducer during a narrow time period after any given ultrasonicpulse. The general concept is illustrated in FIG. 5. The typicalwaveform of a single pulse received by the receive transducer 21 andimposed on the signal line 57 is indicated by a function “f”. The samepulse is repetitively received as an excitation pulse and isrepetitively launched. The received pulse is sampled at a sequence oftime periods labeled t₀-t₁₀. In other words, rather than trying to do areal-time analog to digital conversion of the signal f, the signal issampled during individual fixed time periods t₀-t₁₀ after the transmitpulse is imposed, the analog value during each time period is convertedto a digital function, and that data is stored. Thus the total analogwaveform response can be recreated from the individual digital valuescreated during each time period t, with the overall fidelity of therecreation of the waveform dependent on the number of time periods twhich are sampled. The sampling is not accomplished during a single realtime pulse from the receive transducer 21. Instead, a series of pulsesare emitted from the transmit transducer 21. The external timer isconstructed to provide signals to the sampling A/D converter 42 alongsignal lines 51 and 52 such that the analog value sampled at time periodt₀ when the first pulse is applied to a given transducer element, thenat time t₁ during the second pulse, time t₂ during the third pulse, etc.until all the time periods are sampled. Only after the complete waveformhas been sampled for each element is the next element, i.e. element b,selected. The output from the A/D converter 42 is provided both to themicroprocessor 38 and to the signal processor 41. Thus the digitaloutput values representing the complex waveform f of FIG. 5 can beprocessed by the signal processor 41 after they are compiled for eachtransducer element. The waveform can then be analyzed for time delay orattenuation for any given frequency component with respect to thecharacteristic of the transmitted ultrasonic pulse. The process is thenrepeated for the other elements until all elements have been utilized totransmit a series of pulses sufficient to create digital datarepresenting the waveform which was received at the receive transducerarray 21. It is this data which may then be utilized in a variety ofmethods for determining the physical properties of the member. Dependingon the manner in which the densitometer is being utilized and the databeing sought, the appropriate output can be provided from either themicroprocessor 38 or the signal processor 41 through the digital display18.

Because the ultrasonic pulsing and sampling can be performed so rapidly,at least in human terms, the process of creating a sampled ultrasonicreceived pulse can optionally be repeated several times to reduce noiseby signal averaging. If this option is to be implemented, the process ofrepetitively launching ultrasonic pulses and sampling the receivedwaveform as illustrated in FIG. 5 is repeated one or more times for eachelement in the array before proceeding to the next element. Then thesampled waveforms thus produced can be digitally averaged to produce acomposite waveform that will have a lesser random noise component thanany single sampled waveform. The number of repetitions necessary tosufficiently reduce noise can be determined by testing in a fashionknown to one skilled in the art.

Having thus reviewed the internal operation of the densitometer of FIGS.1-4, it is now possible to understand the methods of use of thedensitometer to measure the physical properties of the member. The firstmethod of use involves measuring transit time of an ultrasonic pulsethrough a subject and comparing that time to the, time an ultrasonicpulse requires to travel an equal distance in a substance of knownacoustic properties such as water. To use the densitometer in thisprocedure, the adjustable arm 16 is adjusted until the member of thesubject, such as the heel, is clamped between the transducers 21. Thenthe knob 19 is tightened to fix the adjustable arm in place. Theactuator button 12 is then pressed to initiate a pulse and measurement.Next the densitometer is removed from the subject while keeping the knob19 tight so that the distance between the transducers 21 remains thesame. The device 10 is then placed about or immersed in a standardmaterial 32 with known acoustic properties, such as by immersion in abath of distilled water. The actuator button 12 is pressed again so thatacoustic signals are transmitted from the transmit transducer 21 throughthe material 32 to the receive transducer 21. While it is advantageousto utilize the whole array of elements a through l for the measurementof the member, it may only be necessary to use a single pair of elementsfor the measurement through the standard assuming only that the standardis homogeneous, unlike the member. The signal profiles received by thetwo measurements are then analyzed by the microprocessor 38 and thesignal processor 41. This analysis can be directed both to thecomparative time of transit of the pulse through the subject as comparedto the standard and to the characteristics of the waveform in frequencyresponse and attenuation through the subject as compared to thestandard.

Thus in this method the densitometer may determine the physicalproperties and integrity of the member 32 by both or either of two formsof analysis. The densitometer may compare the transit time of theacoustic signals through the member with the transmit time of theacoustic signals through the material of known acoustic properties,and/or the device 10 may compare the attenuation as a function offrequency of the broadband acoustic signals through the member 32 withthe attenuation of corresponding specific frequency components of theacoustic signals through the material of known acoustic properties. The“attenuation” of an acoustic signal through a substance is thediminution of the ultrasonic waveform from the propagation througheither the subject or the standard. The theory and experiments usingboth of these methods are presented and discussed in Rossman, P. J.,Measurements of Ultrasonic Velocity and Attenuation In The Human OsCalcis and Their Relationships to Photon Absorptiometry Bone MineralMeasurements (1987) (a thesis submitted in partial fulfillment of therequirements for the degree of Master of Science at the University ofWisconsin-Madison). Tests have indicated that there exists a linearrelationship between ultrasonic attenuation (measured in decibels) (dB))at specific frequencies, and those frequencies. The slope (dB/MHz) ofthe linear relationship, referred to as the broadband ultrasonicattenuation, is dependent upon the physical properties and integrity ofthe substance being tested. With a bone, the slope of the linearrelationship would be dependent upon the bone mineral density. Thusbroadband ultrasonic attenuation through a bone is a parameter directlyrelated to the quality of the cancellous bone matrix.

The microprocessor 38 may therefore be programmed so that the devicedetermines the physical properties and integrity of the member bycomparing either relative transit times and/or relative broadbandultrasonic attenuation through the member and a material of knownacoustic properties. When comparing the transit times, themicroprocessor 38 may be programmed most simply so that the electronics,having received the acoustic signals after they have been transmittedthrough the member, determines the “member” transit time of thoseacoustic signals through the member, and after the acoustic signals havebeen transmitted through the material of known acoustic properties,determines the “material” transit time of the acoustic signals throughthe material. These time periods may be measured most simply by countingthe number of clock pulses of known frequency emitted by the timer 43between the time of launching the pulse and the sensing of the receivedpulse at the A/D converter 42. The microprocessor 38 then makes amathematical “time” comparison of the member transit time to thematerial transit time and then relates that mathematical time comparisonto the physical properties and integrity of the member. The mathematicaltime comparison may be made by either determining a difference betweenthe member transit time and the material transit time, or by determininga ratio between the member transit time and the material transit time.

As a second method of using the densitometer, it may also determine thephysical properties and integrity of the member 32 by determining andcomparing the attenuation of the broadband frequency components of theacoustic signals through the member without reference to a materialhaving known acoustic properties. Using this method, the comparison ofvelocity to a standard is not necessary and absolute transit time of thepulse need not be calculated since it is attenuation that is measured.In such a mode, it is preferable that the transmit transducer 21transmits an acoustic signal which has a broad range of frequencycomponents, such as a simple ultrasonic pulse. In any case, the acousticsignal should have at least one specific frequency component.

In this attenuation comparison mode, the microprocessor 38 is programmedso that after the receive transducer 21 receives the acoustic signalstransmitted through the bone member 32, it determines the absoluteattenuation through the member 32 of the frequency component spectrum ofthe acoustic signals. It is to facilitate the measurement of attenuationthat the excitation amplifier circuit 55 and the receiver amplifier 59have amplification levels which may be digitally controlled. Bysuccessively varying the gain of the amplifiers 55 and 59 on successivepulses, the circuit of FIG. 4 can determine what level of gain isnecessary to place the peak of the received waveform at a proper voltagelevel. This gain is, of course, a function of the level of attenuationof the acoustic pulse during transit through the member 32. After thereceive transducer 21 receives acoustic signals, microprocessor 38 inconjunction with the signal processor 41 determines the absoluteattenuation of individual specific frequency components of the receivedacoustic signal transmitted through the material. The digital signalprocessor 41 then makes mathematical “attenuation” comparisons of thecorresponding individual specific frequency components through themember. A set of mathematical attenuation comparisons betweencorresponding frequency components may be thereby obtained, onecomparison for each frequency component compared. The manner in whichthe attenuation functions with respect to frequency can thus be derived.The microprocessor 38 and digital signal processor 41 then relate thatfunction to the physical properties and integrity of the member.

Shown in FIG. 7 is a sample broadband ultrasonic pulse and a typicalreceived waveform. To achieve an ultrasonic signal that is very broad inthe frequency domain, i.e., a broadband transmitted signal, anelectronic pulse such as indicated at 70 is applied to the selectedultrasonic transducer in the transmit array 21 which then resonates witha broadband ultrasonic emission. The received signal, such as indicatedat 72 in FIG. 7 in a time domain signal plot, is then processed bydiscrete Fourier transform analysis so that it is converted to thefrequency domain. Shown in FIG. 8 is a pair of plots of sample receivedsignals, in frequency domain plots, showing the shift in received signalintensity as a function of frequency between a reference object and aplug of neoprene placed in the instrument. FIG. 9 illustrates a similarcomparison, with FIG. 8 using relative attenuation in the verticaldimension and FIG. 9 using power of the received signal using a similarreference material. Both representations illustrate the difference inrelative intensities as a function of frequency illustrating howbroadband ultrasonic attenuation varies from object to object. Theactual value calculated, broadband ultrasonic attenuation, is calculatedby first comparing the received signal against the reference signal,then performing the discrete Fourier transform to convert to frequencydomain, then performing a linear regression of the difference inattenuation slope to derive broadband ultrasonic attenuation.

The mathematics of the discrete Fourier transform are such that anotherparameter related to bone member density may be calculated in additionto, or in substitution for, broadband attenuation (sometimes referred toas “attenuation” or “BUA” below). When the discrete Fourier transform isperformed on the time-domain signal, the solution for each pointincludes a real member component and an imaginary member component. Thevalues graphed in FIGS. 8 and 9 are the amplitude of the received pulseas determined from this discrete Fourier transform by taking the squareroot of the sum of the squares of the real component and the imaginarycomponent. The phase angle of the change in phase of the ultrasonicpulse as it passed through the member can be calculated by taking thearctangent of the ratio of the imaginary to the real components. Thisphase angle value is also calculated to bone member density.

The microprocessor 38 may also be programmed so that the densitometersimultaneously performs both functions, i.e. determines both transittime and absolute attenuation of the transmitted acoustic signals, firstthrough the member and then through the material with known acousticproperties. The densitometer may then both derive the broadbandultrasonic attenuation function and make a mathematical time comparisonof the member transit time to the material transit time. Themicroprocessor 38 and digital signal processor 41 then relate both thetime comparison along with the attenuation function to the physicalproperties and integrity, or density of the member 32.

In yet another possible mode of operation, the microprocessor 38 may beprogrammed so that the densitometer 10 operates in a mode whereby theneed for calculating either the relative transit time or the attenuationof the acoustic signals through a material of known acoustic propertiesis eliminated. In order to operate in such a mode, the microprocessor 38would include a database of normal absolute transit times which arebased upon such factors as the age, height, weight, race or the sex ofthe individual being tested as well as the distance between thetransducers or the thickness or size of the member. This database ofnormal transit times can be stored in the non-volatile memory or couldbe stored in other media. When testing an individual in this mode, therelevant factors for the individual are placed into the microprocessor38 to select the pertinent normal transit time based on those factors.The transducers 21 are placed on the bone member being tested asdescribed above. When the actuator button 12 is pressed, the acousticsignals are transmitted through the member 32. The receive transducer 21receives those signals after they have been transmitted through themember, and the electronics 31 then determine the “member” transit timeof the acoustic signals through the member. The microprocessor 38 anddigital signal processor 41 then make a mathematical comparison of themeasured member transit time to the selected database normal transittime, and relate the mathematical time comparison to the physicalproperties and integrity, or density of the member, which is displayed.

As an alternative output of the densitometer of the present invention,the digital display 18 could also include a display corresponding to thepattern of the array of elements on the face of the transducer 21 asseen in FIG. 3. This display could then display, for each element athrough l, a gray scale image proportional to the parameter, i.e.transit time or attenuation, being measured. This image may provide avisual indication to an experienced clinician as to the physicalproperties of the member present in the patient.

Shown in FIG. 6 is a circuit schematic for an alternative embodiment ofan ultrasonic densitometer constructed in accordance with the presentinvention. In the circuit of FIG. 6, parts having similar structure andfunction to their corresponding parts in FIG. 4 are indicated withsimilar reference numerals.

The embodiment of FIG. 6 is intended to function with only a singletransducer array 21 which functions both as the transmit and the receivetransducer array. An optional reflecting surface 64 may be placed on theopposite side of the member 32 from the transducer array 21. A digitallycontrolled multiple pole switch 66, preferably an electronic switchrather than a mechanical one, connects the input to and output from theelements of the transducer array 21 selectively either to the excitationamplifier 55 or to the controllable gain receiver/amplifier circuit 59.The switch 66 is connected by a switch control line 68 to an output ofthe microprocessor 38.

In the operation of the circuit of FIG. 6, it functions in most respectslike the circuit of FIG. 4, so only the differences need be discussed.During the launching of an ultrasonic pulse, the microprocessor 38causes a signal to appear on the switch control line 68 to cause theswitch 66 to connect the output of the excitation amplifier 55 to theselected element in the transducer array 21. Following completion of thelaunching of the pulse, the microprocessor 38 changes the signal on theswitch control line 68 to operate the switch 66 to connect the selectedelement or elements as an input to the amplifier 59. Meanwhile, thepulse propagates through the member 32. As the pulse transits throughthe member, reflective pulses will be generated as the pulse crossesinterfaces of differing materials in the member and, in particular, asthe pulse exits the member into the air at the opposite side of themember. If the transition from the member to air does not produce asufficient reflective pulse, the reflecting surface 64 can be placedagainst the opposite side of the member to provide an enhanced reflectedpulse.

The embodiment of FIG. 6 can thus be used to analyze the physicalproperties and integrity of a member using only one transducer 21. Allof the methods described above for such measurements may be used equallyeffectively with this version of the device. The transit time of thepulse through the member can be measured simply by measuring the timeperiod until receipt of the reflected pulse, and then simply dividing bytwo. This time period can be compared to the transit time, over asimilar distance, through a standard medium such as water. The timeperiod for receipt of the reflected pulse could also be simply comparedto standard values for age, sex, etc. Attenuation measurements to detectdifferential frequency measurement can be directly made on the reflectedpulse. If no reflecting surface 64 is used, and it is desired todetermine absolute transit time, the thickness of the member or samplecan be measured.

The use of the multi-element ultrasonic transducer array for thetransducers 21, as illustrated in FIG. 3, enables another advantageousfeature of the instrument of FIGS. 1-9. In using prior artdensitometers, it was often necessary to precisely position theinstrument relative to the body member of the patient being measured tohave useful results. The difficulty arises because of heterogeneities inthe bone mass and structure of actual body members. A measurement takenat one location of density may be significantly different from ameasurement taken close by. Therefore prior art instruments fixed thebody member precisely so that the measurement could be taken at theprecise location each time.

The use of the ultrasonic transducer array obviates the need for thisprecise positioning. Using the instrument of FIGS. 1-9, the instrumentperforms a pulse and response, performs the discrete Fourier transform,and generates a value for broadband ultrasonic attenuation for each pairof transducer elements a through l. Then the microprocessor 38 analyzesthe resulting array of bone ultrasonic density measurements toreproducibly identify the same region of interest each time. In otherwords, since the physical array of transducers is large enough toreliably cover at least the one common region of interest each time, themeasurement is localized at the same locus each time by electricallyselecting the proper location for the measurement from among thelocations measured by the array. The instrument of FIGS. 1-9 isconveniently used by measuring the density of the os calcis as measuredthrough the heel of a human patient. When used in this location, it hasbeen found that a region of interest in the os calcis can be locatedreliably and repeatedly based on the comparisons of broadband ultrasonicattenuation at the points in the array. The region of interest in the oscalcis is identified as a local or relative minimum in broadbandultrasonic attenuation and/or velocity closely adjacent the region ofhighest attenuation values in the body member. Thus repetitivemeasurements of the broadband ultrasonic attenuation value at this sameregion of interest can be reproducibly taken even though thedensitometer instrument 10 is only generally positioned at the samelocation for each successive measurement.

This technique of using a multiple element array to avoid positioncriticality is applicable to other techniques other than thedetermination of broadband ultrasonic attenuation as described here. Theconcept of using an array and comparing the array of results todetermine measurement locus would be equally applicable to measurementstaken of member-density based on speed of sound transit time, othermeasurements of attenuation or on the calculation of phase anglediscussed above. The use of such a multiple-element array, withautomated selection of one element in the region of interest, can alsobe applied to other measurement techniques useful for generatingparameters related to bone member density, such as measuring speedchanges in the transmitted pulse such as suggested in U.S. Pat. No.4,361,154 to Pratt, or measuring the frequency of a “sing-around”self-triggering pulse as suggested in U.S. Pat. No. 3,847,141 to Hoop.The concept which permits the position independence feature is that ofan array of measurements generating an array of data points from which aregion of interest is selected by a reproducible criterion or severalcriteria. The number of elements in the array also clearly can be variedwith a larger number of elements resulting in a greater accuracy inidentifying the same region of interest.

In this way, the ultrasound densitometer of the present inventionprovides a device capable of rapid and efficient determination of thephysical properties of a member in vivo without the use of radiation.Because the densitometer is constructed to operate under the control ofthe microprocessor 38, it can be programmed to operate in one of severalmodes, as discussed above. This allows both for flexibility to clinicalgoals as well as efficient use of the device.

Basin Embodiment

Shown in FIG. 10 is another variation on an ultrasonic densitometerconstructed in accordance with the present invention. In thedensitometer 100 of FIG. 10, there are two ultrasonic transducer arrays121, which are generally similar to the ultrasonic transducer arrays 21of the embodiment of FIG. 1, except that the transducer arrays 21 arefixed in position rather than movable.

The densitometer 100 includes a generally box-shaped mounting case 101with sloping upper face 102 in which is formed a basin 103. The basin103 is sized to receive a human foot and is generally trigonous along avertical plane aligned with the length of the foot so that when the footis placed within the basin 103, the toes of the foot are slightlyelevated with respect to the heel of the foot.

The transducer arrays 121 are positioned in the case 101 so that theyextend into the basin 103 to be on opposite sides of the heel of thefoot placed in the basin 103. When the foot is in position within thebasin 103, the sole of the foot may rest directly on a bottom 104 of thebasin 103 with the heel of the foot received within a curved pocket 106forming a back wall of the basin 103. As so positioned, the transducerarrays 121 are on either side of the os calcis. It has been demonstratedthat placing the transducer approximately 4 centimeters up from the soleand 3.5 centimeters forwardly from the rearward edge of the heel placesthe transducers in the desired region and focused on the os calcis.

The foot may, alternatively, rest on a generally planar foot plate 108having a contour conforming to the bottom 104 and placed against thebottom 104 between the foot and the bottom 104. The foot plate 108 holdsan upwardly extending toe peg 110 for use in reducing motion of the footduring the measurement process. Referring to FIG. 11, the toe peg 110 issized to fit between the big toe and the next adjacent toe of a typicalhuman foot and is mounted in a slot 112 so as to be adjustable generallyalong the length of the foot to accommodate the particular length of thefoot.

The slot 112 cants inward toward a medial axis 114 of the foot, definedalong the foot's length, as one moves along the slot 112 towards theportion of the foot plate 108 near the heel of the foot. This cantingreflects the general relation between foot length and width and allowssimple adjustment for both dimensions at once.

The toe peg 110 is sized to fit loosely between the toes of the footwithout discomfort and does not completely prevent voluntary movement ofthe foot. Nevertheless, it has been found that the tactile feedback tothe patient provided by the toe peg 110 significantly reduces footmovement during operation of the densitometer 100. Two different footplates 108, being mirror images of each other, are used for the left andright foot.

Referring to FIG. 12, the toe peg 110 is held to the slot 112 by afastener 111 having a threaded portion which engages correspondingthreads in the toe peg 110. The head of the threaded fastener 111engages the slot 112 so as to resist rotation. Thus, the toe peg 110 maybe fixed at any position along the length of the slot 112 by simplyturning the toe peg 110 slightly about its axis to tighten the threadedfastener 111 against the foot plate 108.

Referring again to FIG. 10, the basin 103 of the densitometer 110 isflanked, on the upper face 102 of the enclosure 101, by two foot restareas 116 and 118 on the left and right side respectively. Forexamination of a patient's right foot, the patient's left foot may reston foot rest area 118 while the patient's right foot may be placedwithin basin 103. Conversely, for examination of the patient's leftfoot, the left foot of the patient is placed within basin 103 and thepatient's right foot may rest on foot rest area 116. The foot rest areashave a slope conforming to that of the upper face 102 and approximatelythat of bottom 104. The flanking foot rest areas 116 and 118 allow thedensitometer 100 to be used in comfort by a seated patient.

When the densitometer 100 is not in use, the basin area 103 is coveredwith a generally planar cover 120 hinged along the lower edge of thebasin 103 to move between a closed position substantially within theplane of the upper face 102 and covering the basin 103, and an openposition with the plane of the cover 120 forming an angle α with thebottom 104 of the basin 103 as held by hinge stops 122. The angle α isapproximately 90° and selected so as to comfortably support the calf ofthe patient when the patient's foot is in place within basin 103. Tothat end, the upper surface of the cover 120, when the cover 120 is inthe open position, forms a curved trough to receive a typical calf.

The support of the patient's calf provided by the cover 120 has beenfound to reduce foot motion during operation of the densitometer 100.

Referring now to FIGS. 10 and 12, because the densitometer 100 employsfixed transducers 121, a coupling liquid is provided in the basin 103 toprovide a low loss path for acoustic energy between the transducers 121and the patient's foot regardless of the dimensions of the latter. Thecoupling liquid is preferably water plus a surfactant, the latter whichhas been found to improve the signal quality and consistency of thereading of the densitometer. The surfactant may be, for example, acommercially available detergent. It will be recognized, however, thatother flowable, acoustically conductive media may be used to provideacoustic coupling, and hence, that the term “coupling liquid” should beconsidered to embrace materials having a viscosity higher than that ofwater such as, for example, water based slurries and thixotropic gels.

For reasons of hygiene, the exhaustion of the surfactant, and possiblereduction of signal quality with the collection of impurities in thecoupling liquid, it has been determined that the liquid in the basin 103should be changed in between each use of the densitometer 103. Changingthis liquid is time consuming and ordinarily would require convenientaccess to a sink or the like, access which is not always available.Failure to change the liquid may have no immediate visible effect, andhence changing the liquid is easy to forget or delay. For this reason,the present embodiment employs an automated liquid handling systemlinked to the ultrasonic measurement operation through circuitrycontrolled by microprocessor 38 to be described.

Referring to FIG. 13 in the present embodiment, premixed water andsurfactant for filling the basin 103 are contained in a removablepolypropylene supply tank 124, whereas exhausted water and surfactantfrom the basin 103 are received by a similar drain tank 126. Each tank124 and 126 contains a manual valve 128 which is opened when the tanksare installed in the densitometer 100 and closed for transporting thetanks to a remote water supply or drain. The supply tank 124 and thedrain tank 126 have vents 150, at their upper edges as they are normallypositioned, to allow air to be drawn into or expelled from the interiorof the tanks 124 and 126 when they are in their normal position withinthe densitometer 100 and valves 128 are open. The tanks 124 and 126 holdsufficient water for approximately a day's use of the densitometer 100and thus eliminate the need for convenient access to plumbing.

The valve 128 of the supply tank 124 connects the tank through flexibletubing to a pump 130 which may pump liquid from the supply tank 124 to aheating chamber 132.

Referring to FIG. 14, the heating chamber 132 incorporates a resistiveheating element 164 which is supplied with electrical current through athermal protection module in thermal contact with the coupling liquid inthe heating chamber 132. The thermal protection module 166 includes athermostat and a thermal fuse, as will be described below. A thermistor168, also in thermal communication with the liquid in the heatingchamber, provides a measure of the liquid's temperature during operationof the densitometer 100. The heater chamber 132 additionallyincorporates an optical level sensor 172. The level sensor 172 detectsthe level of liquid in the heating chamber 132 by monitoring changes inthe optical properties of a prism system when the prism is immersed inliquid as opposed to being surrounded by air. The operation of thethermistor 168 and the level sensor 172 will be described further below.

Referring again to FIG. 13, the heating chamber 132 communicates throughan overflow port 134 and flexible tubing to an overflow drain outlet136. The overflow outlet 136 is positioned at the bottom of thedensitometer 100 removed from its internal electronics. The overflowport 134 is positioned above the normal fill height of the heatingchamber 132 as will be described in detail below.

The heating chamber 132 also communicates, through its lowermost point,with an electrically actuated fill valve 138 which provides a path,through flexible tubing, to a fill port 140 positioned in the wall ofbasin 103.

In the opposite wall of the basin 103 is an overflow port 142 whichopens into the basin 103 at a point above the normal fill height of thebasin 103 and which further communicates, through a T-connector 144, tothe drain tank 126.

A drain 146, in the bottom 104 of the basin 103, provides a path to anelectronically actuated drain valve 148. The drain valve 148 operates toallow liquid in the basin 103 to flow through the drain 146 to theT-connector 144 and into the drain tank 126. The overflow port 142 anddrain 146 incorporate screens 152 to prevent debris from clogging thetubing or the drain valve 148 communicating with the drain tank 126.

Referring now to FIGS. 10 and 13, the supply tank 124 and the drain tank126 are positioned within the case 101 of the densitometer 100 andlocated at a height with respect to the basin 103 so that liquid willdrain from the basin 103 into the drain tank 126 solely under theinfluence of gravity and so that gravity alone is not sufficient to fillthe basin 103 from supply tank 124 when fill valve 138 is open. Further,the heating chamber 132 is positioned above the basin 103 so that oncethe heating chamber 132 is filled with liquid by pump 130, the fillingof the basin 103 from the heating chamber 132 may be done solely by theinfluence of gravity. Accordingly, the operation of the densitometer infilling and emptying the basin 103 is simple and extremely quiet.

In those situations where plumbing is readily accessible, either or bothof the supply and drain tanks 124 and 126 may be bypassed and directconnections made to existing drains or supply lines. Specifically, thepump 130 may be replaced with a valve (not shown) connecting the heatingchamber 132 to the water supply line. Conversely, the connection betweenthe T-connector 144 and the drain tank 126 may re-routed to connect theT-connector 144 directly to a drain.

Even with the constant refreshing of the coupling liquid in the basin103 by the liquid handling system of the present invention, the liquidcontacting surfaces of the basin 103, the heating chamber 132, thevalves 138 and 148, and the connecting tubing are susceptible tobacterial colonization and to encrustation by minerals. The coatings ofcolonization or encrustation are potentially unhygienic andunattractive. Sufficient build-up of minerals or bacteria may alsoadversely affect the operation of the densitometer 100 either byrestricting liquid flow through the tubing, by interfering with theoperation of the valves 138 or 148, or by adversely affecting theacoustical properties of the transducer array 121.

For this reason, the densitometer 100 is desirably periodically flushedwith an antibacterial solution and a weak acid, the latter to removemineral build-up. These measures are not always effective or may beforgotten, and hence, in the present invention critical water contactingsurfaces are treated with a superficial antibacterial material which isalso resistant to mineral encrustation. The preferred treatment is theSPI-ARGENT™ surface treatment offered by the Spire Corporation ofBedford, Mass. which consists of an ion beam assisted deposition ofsilver into the treated surfaces. The resulting thin film isbactericidal, fungistatic, biocompatible, and mineral resistant. Theproperties of being both bactericidal and fungistatic are generallytermed infection resistant.

This surface treatment is applied to the water contacting surfaces ofthe basin 103, the heating chamber 132 and the critical movingcomponents of the valves 138 and 148.

Referring now to FIG. 14, the general arrangement of the electricalcomponents of FIG. 4 is unchanged in the ultrasonic densitometer 100 ofFIG. 10 except for the addition of I/O circuitry and circuitry tocontrol the pump 130, valves 138 and 148, and heating chamber 132 of theliquid handling system. In particular, microprocessor 38 nowcommunicates through bus 40 with an I/O module 174, a pump/valve controlcircuit 160 and a heater control circuit 162.

I/O module 174 provides the ability to connect a standard video displayterminal or personal computer to the densitometer 100 for display ofinformation to the user or for subsequent post processing of the dataacquired by the densitometer and thus allows an alternative tomicroprocessor 38 and display 18 for processing and displaying theacquired ultrasound propagation data.

The pump/valve control circuit 160 provides electrical signals to thefill valve 138 and the drain valve 148 for opening or closing each valveunder the control of the microprocessor 38. The pump/valve controlcircuit 160 also provides an electrical signal to the pump 130 to causethe pump to begin pumping water and surfactant from the supply tank 124under the control of microprocessor 38, and receives the signal from thelevel sensor 172 in the heating chamber 132 to aid in the control of thepump 130 and valve 138.

The heater control circuit 162 controls the current received by theresistive heating element 164 and also receives the signal from athermistor 168 in thermal contact with the heating chamber 132. A secondthermistor 170, positioned in basin 103 to be thermal contact with theliquid in that basin 103, is also received by the heater control circuit162.

Referring now to FIGS. 13 and 14, during operation of the densitometer100 and prior to the first patient, the basin 103 will be empty, thesupply tank 124 will be filled and contain a known volume of water andsurfactant, and the drain tanks 126 will be empty. Both manual valves128 will be open to allow flow into or out of the respective tanks 124and 126 and the electrically actuated fill valve 138 and drain valve 148will be closed.

Under control of microprocessor 38, the pump/valve control circuit 160provides current to the pump 130 which pumps water and surfactant upwardinto heating chamber 132 until a signal is received from level sensor172. When the heating chamber 132 is filled to the proper level asindicated by level sensor 172, the signal from level sensor 172 topump/valve control circuit 160 causes the pump 130 to be turned off. Atthis time, a predetermined volume of liquid is contained in heatingchamber 132 which translates to the proper volume needed to fill basin103 for measurement.

Under command of microprocessor 38, the heater control circuit 162provides a current through thermal protection module 166 to resistiveheating element 164. The temperature of the liquid in the heatingchamber 132 is monitored by thermistor 168 and heating continues untilthe liquid is brought to a temperature of approximately 39° C. Thethermistor and a thermal fuse (not shown) of the thermal protectionmodule 166 provide additional protection against overheating of theliquid. The thermistor opens at 50° C. and resets automatically as itcools and the thermal fuse opens at 66° C. but does not reset and mustbe replaced. The opening of either the thermistor or the thermal fuseinterrupts current to the resistive heating element 164.

When the liquid in the heating chamber 132 is brought to the correcttemperature, fill valve 138 is opened by microprocessor 38, throughpump/valve control circuit 160, and liquid flows under the influence ofgravity into the basin 103 at the proper temperature. The control of thetemperature of the liquid serves to insure the comfort of the patientwhose foot may be in the basin 103 and to decrease any temperatureeffects on the sound transmission of the water and surfactant.

Once the heated liquid has been transferred from the heating chamber 132to the basin 103, the fill valve 138 is closed and the pump 130 isreactivated to refill the heating chamber 132. Thus, fresh liquid forthe next measurement may be heated during the present measurement toeliminate any waiting between subsequent measurements.

With liquid in place within the basin 103, the measurement of the oscalcis by the densitometer 100 may begin. In this respect, the operationof the ultrasonic densitometer of FIG. 10 is similar to that of theembodiment of FIG. 1 except that the order of pulsing and measurementcan be varied. In the apparatus of FIG. 1, the measurement pulse throughthe member was generally performed before the reference pulse throughhomogenous standard, i.e. water. In the densitometer 100 of FIG. 10,since the distance between the transducers 121 is fixed, the referencepulse through the homogenous standard material, which is simply theliquid in basin 103, may be conducted before or after a measurementpulse through a live member is performed. In fact, because thetemperature of the liquid in the basin 103 is held steady by thetemperature control mechanism as described, the standard transmit timemeasurement can be made once for the instrument and thereafter onlymeasurement pulses need be transmitted.

Preferably, the standard transit time measurement is stored as a numberin the memory of microprocessor 38 during the initial calibration of theunit at the place of manufacture or during subsequent recalibrations.During the calibration of the densitometer 100, the signal from thethermistor 170 is used to produce a transit time corrected for thetemperature of the liquid according to well known functional relationslinking the speed of sound in water to water temperature. It is thiscorrected transit time that is stored in the memory associated withmicroprocessor 38 as a stored standard reference.

The transit time of the measurement pulses is compared to the storedstandard reference transit times through the coupling liquid to give anindication of the integrity of the member just measured. Thus, one maydispense with the reference pulse entirely. Empirical tests havedetermined that by proper selection of a standard reference value storedin the memory of microprocessor 38 and by holding the liquid in thebasin within a temperature range as provided by the heating chamber 132,no reference pulse need be launched or measured.

Using this variation, a mathematical comparison of the measured transittime, or transit velocity, must be made to the standard. Since, in theinterests of accuracy, it is preferred to use both changes in transittime (velocity) and changes in attenuation to evaluate a member in vivo,the following formula has been developed to provide a numerical valueindicative of the integrity and mineral density of a bone:

bone integrity value=A(SOS-B)+C(BUA-D)  (1)

In this formula, “SOS” indicates the speed of sound, or velocity, of themeasurement ultrasonic pulse through the member, and is expressed inmeters per second. The speed of sound (SOS) value is calculated from themeasured transit time by dividing a standard value for the member widthby the actual transit time measured. For an adult human heel, it hasbeen found that assuming a standard human heel width of 40 mm at thepoint of measurement results in such sufficient and reproducibleaccuracy that actual measurement of the actual individual heel is notneeded.

BUA is broadband ultrasonic attenuation, as described in greater detailabove. The constants A, B, C, and D offset and scale the influence ofthe BUA measurement relative to the SOS measurement to provide a moreeffective predictor of bone density. These constants may be determinedempirically and may be selected for the particular machine to providenumbers compatible with dual photon absorptiometry devices, such as anestimated bone mineral density (BMD) value, and to reduce bone widtheffects. Since this method utilizing ultrasonic measurement of the heelis quick and free from radiation, it offers a promising alternative forevaluation of bone integrity. It will be understood that multiple SOSand BUA values may be obtained, averaged, then combined per the aboveformula, or that each SOS and BUA value may be combined to produce abone integrity value and the multiple bone integrity values thenaveraged.

The densitometer 100 may be used with or without an array of ultrasonictransducers in the transducers 121. In its simplest form the mechanicalalignment of the heel in the device can be provided by the shape andsize of the basin 103. While the use of an array, and region-of-interestscanning as described above, is most helpful in ensuring a reproducibleand accurate measurement, mechanical placement may be acceptable forclinical utility, in which case only single transducer elements arerequired.

Upon completion of the measurement, the drain valve 148 is opened bymicroprocessor 38 through pump/valve control circuitry 160, and theliquid in the basin 103 is drained through “T” 144 to the drain tank126. At the beginning of the next measurement, the drain valve 148 isclosed and liquid is again transferred from the heating chamber 132 ashas been described.

With repeated fillings and drainings of the basin 103, the level ofliquid in the fill tank 124 decreases with a corresponding increase inthe level of the liquid in the drain tank 126. The height of the liquidin each tank 124 and 126 may be tracked by a conventional level sensorsuch as a mechanical float or a capacitive type level sensor.

Preferably no additional level sensor is employed. The volume of liquidfor each use of the densitometer 100 is known and defined by the filllevel of the heating chamber 132. The microprocessor 38 may thereforetrack the level of liquid remaining in the supply tank 124 by countingthe number of times the basin 103 is filled to provide a signal to theuser, via the display 18 or a remote video display terminal (not shown),indicating that the tanks 124 and 125 need to be refilled and drainedrespectively. This signal to the user is based on the number of timesthe basin 103 is filled and a calculation of the relative volumes of theheating chamber 132 and supply tank 124.

After completion of the use of the densitometer 100 for a period oftime, the densitometer may be stored. In a storage mode, after both thesupply tank 124 and drain tank 126 have been manually emptied, themicroprocessor 38 instructs the pump/valve control circuit 160 to openboth the fill valve 138 and the drain valve 148 and to run the pump 130.The drain valve 138 is opened slightly before the pump 130 is actuatedto prevent the rush of air from causing liquid to flow out of theoverflow port 134.

Referring now to FIGS. 10 and 15, the transducers 121 are inserted intothe basin 103 through tubular sleeves 180 extending outward from thewalls of the basin 103 at the curved pocket along an axes 212 of theopposed transducers 121. The tubular sleeves 180 define a circular borein which the transducers 121 may be positioned. Each transducer 121seals the sleeve 180 by compression of o-ring 182 positioned on theinner surface of the sleeve 180.

Although the transducers 121 fit tightly within the sleeves 180, theirseparation and alignment are determined not by the sleeves 180 but by anindependent C-brace 184 comprising a first and second opposed arm 186separated by a shank 188. A transducers 121 is attached to one end ofeach of the arms 186, the other ends of the arms 186 fitting against theshank 188.

The arms 186 are generally rectangular blocks transversely bored toreceive the cylindrically shaped transducers 121 at one end and to holdthem along axis 212. The other ends of the arms 186 provide planar facesfor abutting the opposite ends of the block like shank 188, the abuttingserving to hold the arms 186 opposed and parallel to each other.

Although the angles of the arms 186 with respect to the shank 188 aredetermined by the abutment of the planar faces of the arms 186 and theends of the shank 188, alignment of the arms 186 with respect to theshank 188 is provided by dowel tubes 190 extending outward from each endof the shank 188 to fit tightly within corresponding bores in the firstand second arm 186.

Cap screws 194 received in counterbored holes in the arms 186 passthrough the arms 186, the dowel tubes 190 are received by threaded holesin the shank 188 to hold the arm 186 firmly attached to the shank 188.The dowel tubes 190 and surfaces between the arms 186 and shank 188serve to provide extremely precise alignment and angulation of thetransducers 121, and yet a joint that may be separated to permit removalof the transducers 121 from the densitometer 10 for replacement orrepair.

Transducers 121 are matched and fitted to the arms 186 in a controlledfactory environment to provide the necessary acoustic signal strengthand reception. In the field, the shank 188 may be separated from one orboth arms 186 by loosening of the cap screws 194 so as to allow thetransducers 121 extending inward from the arms 186 to be fit within thesleeves 180. Proper alignment and angulation of the transducers is thenassured by reattaching the arm or arms 186 removed from the shank 188 tothe shank 188 to be tightened thereto by the cap screws 194. Thus, thealignment of the transducers is not dependent on the alignment of thesleeves 180 which may be molded of plastic and thus be of relatively lowprecision. Nor must alignment be tested while the transducers are in thesleeves 180 attached to the basin 103 but may be checked in a centralcontrolled environment.

Flexible Bladder with Moving Supports

Referring now to FIGS. 16 and 17, in yet another embodiment of thepresent invention, the opposed transducers 121 are fitted with annularcollars 200 which in turn are attached to flexible bladders 202extending inward to the basin 103, each bladder 202 containing a liquidor semi-liquid coupling material 204.

The bladders 202 serve to contain the gel about the face of thetransducers 121 and conform to the left and right sides of a patient'sheel 207, respectively, to provide a path between the transducers 121and the soft tissue and bone of the heel 207 without intervening air.The bladder 202 further prevents the coupling material from directcontact with the heel to permit selection of the coupling material 204from a broader range of materials.

Compression of the bladders 202 against the heel 207, so as to providethe necessary coupling, is provided by a telescoping shank 181 shown inFIG. 16. In this alternative embodiment of the C-brace 184 of FIG. 15,the shank 188′ has been cut into two portions 206 and 208 slidablyconnected together by dowel pins 210 to provide necessary motion of thetransducers 121 inward along their axis to compress the bladders 202against the heel 207. One end of each dowel pin 210 is press fit withinbores in the shank 188′ parallel to the axis 212 of the opposedtransducers in portion 206. The other ends of the dowel pins 210 slidewithin larger bores in portion 208 so that portions 208 and 206 mayslide toward and away from each other parallel to the axis 212. Withsuch motion, the attached arms 186 move towards and away from each otheradjusting the separation of the transducers 121 between an open positionfor insertion of the heel 207 and a closed position of known separationand orientation where portions 208 and 206 abut.

Control of the separation is provided by means of cam pins 214protruding from portions 206 and 208 on the side away from the extensionof the arms 186 and generally perpendicular to the axis 212. These pins214 are received by spiral shaped slots in a cam disk 217 fitting overthe cam pins 214. The disk includes radially extending lever 218 whosemotion rotates the disk causing the cam pins 214 within the slots 215 tobe moved together or apart depending on motion of lever 218.

Thus, the transducers 121 may be moved apart together with the bladders202 for insertion of the heel 207 into the basin 103. Once the heel isin place, motion of the lever 218 closes the transducers 121 to apredetermined fixed separation compressing the bladders 202 snuglyagainst the sides of the heel 207. The elasticity of the bladder filledwith coupling material 204 provides an expanding force against the heel207 to closely conform the surface of the bladder 202 to the heel 207.

The collars 200 may provide a conduit for electrical wiring (not shown)including wiring attached to a temperature sensor for monitoring thetemperature of the contained coupling material 204 and heater elementsfor heating the temperature of the contained coupling material to apredetermined temperature. As has been described above, the controlledheating provides both for comfort to the patient and for reproducibilityof the measurements which may be influenced by the temperature of thecontained coupling material 204.

Cancellation of Heel Width Variations

Referring to FIGS. 17 and 18, generally the thicker the calcaneus 216 ofthe heel 207, the greater the attenuation of an acoustic signal passingthrough the heel 207 between transducers 121. Correspondingly, withgreater attenuation, the slope of attenuation as a function offrequency, generally termed broadband ultrasonic attenuation (BUA) alsoincreases as shown generally in FIG. 18 by plot 209. This assumesgenerally that the coupling medium 204 is of low or essentially constantattenuation as a function of frequency. Greater BUA is generallycorrelated to higher bone quality.

For constant heel thickness, lower TOF (faster sound speed) correspondsgenerally to higher bone quality. The time of flight (TOF) of anacoustic pulse between the transducers 121 will be proportional to thetime of flight of the acoustic pulse through regions A of FIG. 17comprising the path length through coupling gel 204, regions Bcomprising the path length through soft tissue of the heel 207surrounding the calcaneus 216, and region C comprising the path lengththrough the heel bone or calcaneus 216. Thus, $\begin{matrix}{{TOF} = {{\frac{1}{V_{A}}A} + {\frac{1}{V_{B}}B} + {\frac{1}{V_{C}}C}}} & (2)\end{matrix}$

where V_(A), V_(B), and V_(C) are the average speed of sound through thecoupling gel, soft tissue and bone respectively and A, B, C are the pathlengths through these same materials. Provided that the separationbetween the transducers 121 is a constant value K, then time of flightwill equal: $\begin{matrix}{{TOF} = {{\frac{1}{V_{A}}\left( {K - C - B} \right)} + {\frac{1}{V_{B}}B} + {\frac{1}{V_{C}}C}}} & (3)\end{matrix}$

The change in time of flight as a function the thickness of the bone C(the 5 derivative of TOF with respect to C) will thus generally be equalto: $\frac{1}{V_{C}} - {\frac{1}{V_{A}}.}$

Referring now to FIG. 18, if the velocity of sound through the couplingmedium 204 is greater than that through the bone being measured$\left( {{V_{A} > V_{C}},\quad {{{or}\quad \frac{1}{V_{C}}} > \frac{1}{V_{A}}}} \right),$

then the functional relationship of TOF to heel width will be one ofincreasing as the heel becomes wider (indicated at plot 213 showingvalues of 1/TOF). On the other hand, if the velocity of sound throughthe coupling medium 204 is less than that through the bone beingmeasured$\left( {{V_{C} > V_{A}},\quad {{{but}\quad \frac{1}{V_{A}}} > \frac{1}{V_{C}}}} \right),$

then the functional relationship of TOF to heel width will be one ofdecreasing as the heel becomes wider (indicated at plot 211 showingvalues of 1/TOF).

A combined bone quality figure may be obtained by combining BUA and1/TOF measurements (1/TOF because BUA increases but TOF decreases withdenser bone). Further, if (1) the conditions of ultrasonic propagationare adjusted so that the slope of 1/TOF with heel width is opposite insign to the slope of BUA with heel width (i.e., V_(A)>V_(C)) and (2) theBUA and 1/TOF measurements are weighted with respect to each other sothat the opposite slopes of the BUA and 1/TOF are equal, then thealgebraic combination of the BUA and TOF, through addition for example,will produce a bone quality measurement substantially independent ofheel width for a range of bone qualities.

This can be intuitively understood by noting that as the heel getswider, it displaces some of the coupling gel 204 from between the heel207 and each transducer 121, and by displacing material that conductssound slower than the bone being measured increasing the total speedwith which the sound is conducted.

Note that a similar effect may be obtained by proper scaling andcombination of BUA and TOF by multiplication and that other functions ofattenuation and TOF could be used taking advantage of their functionalindependence and their functional dependence in part on heel width.

Referring now to FIG. 19, generally BUA and TOF are functionally relatedto both bone quality and bone width. It should be possible, therefore,to solve the equations governing these relationships for bone qualityalone and thus to eliminate the effect of the common variable of heelwidth. With such an approach, the variable of heel width is eliminatednot just for a portion but through the entire range of bone measurementprovided that the coupling medium is different from the bone beingmeasured so that there will be a width effect in both BUA and TOFmeasurements.

Approximations of the algebraic relationships describing the functionaldependence of BUA and TOF on bone quality and bone width, can beobtained through the construction of a set of bone phantoms of differentwidths and bone qualities when using a particular coupling gel.Generally, for each value of BUA or TOF the data will describe a curve222 linking that value with different combinations of bone quality andbone width. This data may be placed in a look-up table in the memory ofthe microprocessor of the densitometer as has been previously described.

After BUA and TOF values are determined, the data of the look-up table(comprising many bone quality and bone width pairs for each of thedetermined BUA and TOF values) are scanned to find a bone quality andwidth data pair for the BUA value matching a bone quality and width datapair for the TOF value. This is equivalent to finding the intersectionof the two curves 222 associated with the measured BUA and TOF values.The matching bone quality values of the data base will give a bonequality having little or no bone width influence. This value may bedisplayed to the clinician. It is noted that the previously describedtechnique of summing weighted values of BUA and 1/TOF is but aspecialized form of this process of algebraic solution.

Alternatively, a matching bone width value can be identified, being thewidth of the measured heel, and used to correct either of the BUA or TOFvalues for display to the clinician in circumstances where BUA or TOFvalues are preferred for diagnosis.

This ability to cancel out heel width effects will work only for bonequalities where the relationship between the coupling gel 204 and thecalcaneus 216 are such as to provide a functional dependence on heelwidth. Cancellation will not occur, for example, if the density of thecalcaneus 216 being measured is substantially equal to the sound speedof the coupling gel 204 and thus where displacement of the coupling gelby similar bone will have no net effect on time of flight. Thus thecoupling gel must be properly selected. In this case, materials havinghigher sound speed may be selected for the coupling material. Thedifference between the coupling gel and the bone being measured willinfluence the accuracy of the cancellation of heel width effects.

Moderating this desire to improve heel width effects is the importanceof keeping the coupling gel 204 close to the acoustic properties of thesoft tissue of the heel 207 both to prevent reflection by impedancemismatch and to prevent variations in the thickness of the soft tissuein regions B from adding additional uncertainty to the measurement. Thecoupling medium of water provides good matching to the soft tissue ofthe heel 207 and has a sound velocity very close to bone and someosteoporotic conditions. Weighting of the attenuation and propagationtime may be made for water.

Although the preferred embodiment of the invention contemplates displayof a bone quality value or corrected TOF or BUA values, it will berecognized that the same effect might be had by displaying uncorrectedBUA or TOF values on a chart and establishing a threshold for strong orweak bone based on the corrections determined as above.

Ultrasonic Densitometer with Scannable Focus

Referring now to FIG. 20, a receiving transducer array 300, similar toarray 21 described with respect to FIG. 1, may be positioned adjacent tothe heel of a patient (not shown) to receive an ultrasonic wave 410along axis 304. The receiving transducer array 300 includes apiezoelectric sheet 302 of substantially square outline positionednormal to the transmission axis 304 and is divided into transducerelements 400 as will be described, each which receives a differentportion of the ultrasonic wave 410 after passage through the heel.

The piezoelectric sheet 302 may be constructed of polyvinylidenefluoride and has a front face 306 covered with a grid of interconnectedsquare electrodes 308 deposited on the front face by vacuummetallization. These square electrodes 308 are arranged at theinterstices of a rectangular grid to fall in rectilinear rows andcolumns. Referring also to FIG. 23, each square electrode 308 is spacedfrom its neighboring electrodes 308 by approximately its width. Thesesquare electrodes 308 are Connected together by metallized traces (notshown) and to a common voltage reference by means of a lead 310.

In the manufacture of the piezoelectric sheet 302, the polyvinylidenesheet is polarized to create its piezoelectric properties by heating andcooling the sheet in the presence of a polarizing electrical fieldaccording to methods generally understood in the art. In the presentinvention, this polarizing field is applied only to the area under thesquare electrodes 308 so that only this material is piezoelectric andthe material between square electrodes 308 has reduced or nopiezoelectric properties. As will be understood below, this selectivepolarization of the piezoelectric sheet 302 provides improved spatialselectivity in distinguishing between acoustic signals received atdifferent areas on the piezoelectric sheet.

Referring now to FIG. 22, opposite each electrode 308 on the back sideof the piezoelectric sheet 302 furthest from the source of theultrasonic wave 410 is a second electrode 312 having substantially thesame dimensions as the square electrodes 308 and aligned withcorresponding square electrodes 308 along transmission axis 304.

Referring to FIGS. 20 and 22, a connector board 318 of areal dimensionsubstantially equal to the piezoelectric sheet 302 has, extending fromits front surface, a number of conductive pins 320 corresponding to thepads 316 in number and location. The pins 320 are stake-type terminalsmounted to an epoxy glass printed circuit board 322 of a type well knownto those of ordinary skill in the art. Each conductive pin 320 isconnected directly to a preamplifer and then by means of printed circuittraces to a multiplexer 325 to a reduced number of control and datalines 324 which may be connected to the microprocessor 38 of thedensitometer through an A to D converter 42 described previously withrespect to FIG. 1 and as is well understood in the art. The preamplifersallow grounding of those electrodes 312 not active during scanning toreduce cross-talk between electrodes 312.

As shown in FIG. 22, the pins 320 of the connector board 318 areelectrically connected to electrodes 312 on the back surface of thepiezoelectric sheet 302 by means of a strip of thin (0.0005″) Mylar 316having conductive fingers 314 on its surfaces. The conductive fingers314 on the front and rear surfaces of the Mylar strip 316 are inelectrical communication through a plated-through hole 313 in the Mylar316 connecting the fingers 314.

Each conductive pin 320 is attached to a conductive finger 314 at oneedge of the Mylar strip 316 at the rear of the Mylar strip 316(according to the direction of the acoustic wave) by means of ananisotropically conductive adhesive film 315 providing electricalconduction only along its thinnest dimension, thus from pin 320 tofinger 314 but not between fingers 314 or pins 320. Anisotropicallyconductive film suitable for this purpose is commercially available from3M corporation of Minnesota under the trade name of 3M Z-Axis AdhesiveFilm.

The other end of each plated finger 314 on the front of the Mylar strip316 is then connected to an electrode 312 by a second layer ofanisotropically conductive adhesive film 317. The Mylar strip 316 flexesto allows the pins 320 to be spaced away from the electrode 312 toreduce reflections off the pins 320 such as may cause spurious signalsat the piezoelectric sheet 302. The Mylar strip 316 and conductivefingers 314 are essentially transparent to the acoustic wave.

Referring to FIG. 22, the Mylar strips 316 and adhesive film 315 and 317allow rapid assembly of the transducer 300. A single layer of conductivefilm 317 (not shown in FIG. 22) may be applied over the entire rearsurface of the piezoelectric sheet 302 and electrodes 312. Next aplurality of overlapping Mylar strips 316 may be laid down upon thissurface, each Mylar strip 316 extending laterally across thepiezoelectric sheet 302 with transversely extending conductive fingers314 for each electrode 312 of one row of conductive electrode 312. Theoverlapping of the Mylar strips 316 ensures that only a front edge ofeach strip 316 adheres to the piezoelectric sheet 302. Guide holes 319in the laterally extreme edges of the Mylar strips 316 fit into pins ina jig (not shown) to ensure alignment of fingers 314 with electrodes312.

Next, a second layer of the anisotropically conductive adhesive film 315is placed on the rear surfaces of the overlapping Mylar strips 316 andthe conductive pins 320 pressed down on this film 315, aligned with theother ends of the conductive fingers 314 to attach to their respectivefingers 314. The conductive pins 320 are then raised and fixed in spacedapart relationship with the piezoelectric sheet 302, the Mylar strips316 flexing to accommodate this displacement.

The ultrasonic wave 410 passing through portions of the piezoelectricsheet 302 between electrodes 308 and 312 may thereby be measured at anumber of points over the surface of the piezoelectric sheet by theelectric signals generated and collected by electrodes 308 and 312according to multiplexing methods well known in the art. Each electrodepair 308 and 312 provides an independent signal of the acoustic energypassing through the area of the piezoelectric sheet 302 embraced by theelectrode pair.

A protective frame 325 encloses the piezoelectric sheet 302 andconnector board 318 protecting them from direct contact with water ofthe basin 103 shown in FIGS. 10 and 15 into which the receivingtransducer array 300 may be placed. The frame 325 holds on its frontface an acoustically transparent and flexible material 326 such as aTeflon film so that the ultrasonic wave 410 may pass into the frame toreach the piezoelectric sheet 302.

The above described array may be used either to receive or transmitacoustic waves and is not limited to use in the medical area but mayprovide an inexpensive and rugged industrial acoustic array useful for avariety of purposes including industrial ultrasonic imaging and theconstruction of high frequency synthetic aperture microphones.

Positioned behind the frame 325 is an electric motor 328 driving acentral gear 330 about an axis aligned with transmission axis 304 andapproximately centered within the frame 325. The central gear 330 inturn engages two diagonally opposed planet gears 332 also turning aboutaxes aligned with the transmission axis. Each planet gear 332 has a rod334 extending forwardly from a front face of the planet gear 332 butoffset from the planet gear's axis to move in an orbit 336 thereabout.The orbit 336 has a diameter approximately equal to the spacing betweenelectrodes 308.

The rods 334 engage corresponding sockets 338 on the back side of theframe 325 at its opposed comers. Thus activation of the motor 328 causesthe piezoelectric sheet 302 and connector board 318 to follow the orbit336 while maintaining the rows and columns of detector elements 400 inhorizontal and vertical alignment, respectively.

Referring now to FIG. 23, a sampling of the signals from the detectorelements 400 may be made at four points 342 in the orbit 336 at whicheach electrode 308 is first at a starting position, and then is movedhalf the inter-electrode spacing upward, leftward, or upward andleftward. The effect of this motion of the detector elements 400 is todouble the spatial resolution of the received acoustic signals withoutincreasing the amount of wiring or the number of detector elements 400.The sampling of acoustic energy at each of the points 342 is stored inthe memory of the microprocessor and can be independently processed toderive attenuation, BUA or time of flight measurements or a combinationof these measurements. These measurements are then converted to anintensity value of an image so that each pixel of the image has anintensity value proportional to the measured parameter. A clinicianviewing the image thus obtains not merely an image of the bone, but animage that indicates bone quality at its various points.

A transmitting ultrasonic transducer 408 is positioned opposite thereceiving transducer array 300 from the heel 207 and produces agenerally planar ultrasonic wave 410 passing into the heel. Generally,the acoustic signal received by each transducer element 400 will havearrived from many points of the heel.

Referring now to FIG. 24, if the transducer elements 400 were focused asindicated by depicted transducer elements 400′ to follow a hemisphere402 having a radius and hence focus at a particular volume element orvoxel 404 within the heel, acoustic signals from other voxels could becanceled providing greater selectivity in the measurement. In thisfocusing of the transducer elements 400′, the signals from each of theelements 400′ are summed together. Constructive and destructiveinterference of ultrasonic waves 410 from the heel 207 serve toeliminate acoustic signals not flowing directly from focus volumeelement 404.

For example as depicted, two acoustic signals 405 and 406 from focusvoxel 404 both crest at the location of a transducer element 400′ as aresult of the equidistance of each transducer element 400′ from focusvoxel 404. When the signals from transducer elements 400′ are summed,the signal from focus voxel 404 will increase. In contrast, acousticsignals from other voxels not equidistant to transducer elements 400′will tend to cancel each other when summed and thus decrease.

The present invention does not curve the transducer elements 400 into ahemisphere but accomplishes the same effect while retaining thetransducer element 400 in a planar array by delaying the signalsreceived by the transducer elements 400 as one moves toward thecentermost transducer element 400″ so as to produce an effectivehemispherical array. Like a hemispherical array, the center-mosttransducer elements 400 appear to receive the acoustic wave a littlelater than the transducer elements 400 at the edge of the receivingtransducer array 300. By using a phase delay of the signals instead ofcurving the receiving array 300, the position of the focus voxel 404 atwhich the receiving array 300 is focused, may be scanned electrically aswill be described. The signals from each of the transducer elements 400are received by the A/D converter 42 and stored in memory. Phaseshifting as described simply involves shifting the point at which onestarts reading the stored signals.

Adjusting the phase of the acoustic signals received by each of thetransducer elements 400 allows the location of the focus voxel 404 fromwhich data is obtained to be scanned through the heel. The phase issimply adjusted so that the effective arrival time of an acoustic signaloriginating at the desired location is the same for each of thetransducer elements 400.

Referring now to FIG. 25, the location of focus voxel 404 may be movedin a first and second raster scan pattern 412 and 414 (as readings aretaken over many ultrasonic pulses) to obtain separated planes of datanormal to the transmission axis 304. The first plane of data 412 may,for example, be positioned near the outer edge of the os calcis 216 tomeasure the cortical bone quality while the second plane 414 may beplaced in a centered position in the trabecular bone to obtain asomewhat different reading, both readings providing distinct data aboutthe bone.

It will be understood that this same approach of scanning in differentplanes may be used to obtain a volume of data within the heel 207, inthis case, the focus voxel 404 being moved to points on a threedimensional grid.

In another embodiment (not shown) the transmitting ultrasonic transducermay be an array and the phases of the ultrasonic signals transmitted byeach of the elements of the array may be phased so as to focus on aparticular voxel within the heel. In this case, the receiving array maybe a single broad area detector or may also be an array focused on thesame voxel for increased selectivity. The focus point of thetransmitting and receiving arrays may also be shifted with respect toeach other to investigate local sound transfer phenomenon. As before,the focal points of either array may be steered electrically by themicroprocessor through a shifting of the phases of the transmitted andreceived signals. To collect data, each element of the transmit arraymay be energized individually while all receive elements of the receivearray are read. This may be continued until each of the elements of thetransmit array have been energized.

Alternatively, referring to FIG. 28, the receiving array 300 may beactually formed so that its elements follow along the hemisphere 402 soas to have a fixed focus on focus voxel 404. Additional circuitry toeffect the phase adjustment needed to focus the array is not needed inthis case. The receiving array 300 is attached to an X-Y-Z table 600providing motion in each of three Cartesian axes under the control ofthe microprocessor via stepper motors 610. At each different location ofthe table 600, data may be collected from focus voxel 404 to establishthe data points on the three dimensional grid. The transmitting array408 may be held stationary or may be moved with the scanning of thereceiving array 300 and may be focused as well.

Referring now to FIG. 26, such a data volume 415 may include a pluralityof data voxels 416 each providing a measured member parameter for thebone or tissue at that point in the heel. A point of minimum bonedensity 418 may be found within this data volume 415 and used toidentify a region of interest 420 which will serve as a standard regionfor measuring the bone density of the heel. This region may beautomatically found after collection of the data volume 415 and onlythose voxels 416 within the region of interest 420 may be used for adisplayed measurement. This automatic location of a region of interest420 provides a much more precise bone characterization.

Acquiring a data volume 415 also provides the opportunity to use theextra data outside the region of interest 420 to ensure that the sameregion of interest 420 is measured in the patient's heel over a seriesof measurements made at different times. The data volume 415 may bestored in memory as a template that may be matched to subsequentlyacquired data volumes. The region of interest 420 spatially located withrespect to the first template, may then be used as the region ofinterest for the subsequent data volumes aligned with that template toprovide more repeatability in the measurement.

Referring now to FIG. 27 in such a template system in a first step 500,a collection of a data volume 415 within the heel is obtained. Atdecision block 502, if this is a first measurement of a particularpatient, a region of interest 420 is identified at process block 504from this data, as a predetermined volume centered about a point ofminimum bone density 418 as described with respect to FIG. 26. Atprocess block 506, the data volume is stored as a template along withthe region of interest defined with respect to the data of the template.

Referring again to decision block 502 on a subsequent measurement of apatient, the program may proceed to process block 508 and the templatepreviously established may be correlated to a new data volume 415collected at process block 500. The correlation process involvesshifting the relative locations of the two data volumes to minimize adifference between the values of each data voxel 416 of the datavolumes. In most situations, this will accurately align the two datavolumes so that corresponding voxels 416 of the two data volumes 415measure identical points within the patient's heel. The region ofinterest 420 associated with the template is then transferred to the newdata volume as it has been shifted into alignment with the template sothat the identical region of interest may be measured in a patient evenif the patient's foot has taken a different alignment with respect tothe transducer array 300 and 408. This use of the template's region ofinterest 420 is indicated by process block 510.

At process block 512, an index is calculated at the region of interest420 for the new data volume 415 being typically an average value of abone parameter such as BUA or time of flight for the voxels 416 withinthe region of interest 420. This data is then displayed to the clinicianat process block 520 as has been described.

Overlapping Bladder Embodiment

Referring now to FIG. 29, in an alternative embodiment, a transducerarray 300, as has been described above, or an array 21, described withrespect to FIG. 3, may extend into the basin 103 which is no longerfilled with water. Instead, the transducer array 300 is coupled to theheel 207 through three concentric overlapping bladder walls.

The first, internal bladder 522 is supported about the transducer array300 by means of an annular collar 524 to provide an enclosed volume inmuch the same manner as has been described with respect to the bladder202 of FIG. 17 with the exception that the annular collar 524 includesan orifice 526 allowing a first coupling liquid to be introduced intothe bladder 522 through delivery pipe 528 as driven by pump 130B. Abranch connection 530 to delivery pipe 528 provides the same couplingliquid to an identical and opposing transducer/bladder assembly on theother side of the heel, not shown in FIG. 29.

A heat exchanger 532, as is well understood in the art, couples to thedelivery pipe 528 to ensure that the coupling liquid 534 is at aconstant predetermined temperature to which the device has beenpreviously calibrated. The heat exchanger 532 may make use of acirculating exchanger liquid preheated to a constant temperature bymeans of a combination thermostat and electric heater (not shown) wellknown in the art. A pressure transducer 536 communicating with thedelivery pipe 528 measures the pressure of the coupling liquid 534 andis used to control the pump 130B so that the bladder 522 is inflatedonly to a predetermined pressure. This ensures a repeatable andpredetermined deformation of the soft tissue of the heel 207 andprovides, to the extent possible, a reproducible and constant couplingbetween the transducer array 300 and the, heel 207.

Coaxially about annular collar 524 is a second annular collar 538supporting on its outer edge an second bladder 540 providing a secondenclosed volume between itself and bladder 524 to hold a second couplingliquid 542. Second coupling liquid 542 differs from coupling liquid 534by having either significantly different ultrasonic attenuationcharacteristics or a significantly different sound speed characteristicsfor reasons as will be describe below. The annular collar 538 holdingthe second bladder 540 is also pierced by an orifice 544 connecting to adelivery pipe 546, which like delivery pipe 528, passes through a heatexchanger 550 supplied with the same liquid at the same temperature asheat exchanger 532. Delivery pipe 546 also communicates with pressuretransducer 536 through a coupler 552 which prevents intermingling of theliquids in delivery pipe 528 and 546. Delivery pipe 546 includes abranch connection 554, which like branch connection 530, provides acomparable liquid at a comparable temperature to the opposed transducernot shown in FIG. 29. A separate pump 130A delivers the coupling liquid542 from a distinct reservoir from that connected to pump 130B.

The bladders 522 and 540 for both transducers are sufficiently flexibleand have ample size so that either bladder 522 or 540 through inflationto a pressure less than the predetermined pressure controlled bypressure transducer 536 will bridge any gap between the transducer array300 and the heel 207, thus allowing the transducer arrays 300 to befixed in separation. This both simplifies mechanical construction andeliminates errors resulting from uncertainty about the separationdistance of the transducer arrays 300 as is inherent in any movablesystem.

Bladder Replacement Mechanism

Referring now to FIGS. 29 and 32, outside of annular collar 538 is yetanother annular locking collar 556 mounted along with the other annularcollars 538 and 524 against one wall of the basin 103 about thetransducer arrays 300. Locking collar 556 includes radially outwardextending tabs 558 which engage notches 560 in a stretcher ring 562which may be installed against and removed from locking collar 556 bypassing the notches 560 over the tabs 558 and giving the stretcher ring562 a partial clockwise turn to move the locking tabs 558 into undercutportions 564, thereby retaining stretcher ring 562 against the wall ofthe wall of basin 103 against outward forces away from the transducerarray 300. The stretcher ring is covered with a third flexible bladder566 which envelopes both bladders 522 and 540 and yet which is easilyremoved and may be disposed of to provide for hygienic reuse of bladder540 and 522. Because the third flexible bladder 566 is not required toretain a liquid and is disposable, it may be made from a lighter andless resilient material.

Second Width Correction and Extrapolation of Measurement

The apparatus of FIG. 29 is controlled by means of the microprocessor 38(previously described and shown in FIG. 14) having connections to thepressure transducer 536 and pumps 130A and 130B as indicated in FIG. 33.Referring now to FIG. 33, at a beginning of a measurement session, asindicated by process block 568, pump 130B is activated to inflate innerbladders 522 of opposed transducer arrays 300, as shown in FIG. 30, withliquid A to a constant pressure, thus extending the bladders 522 outwardagainst the sides of the heel to oppose the os calcis 216. Any liquid inbladder 540 is withdrawn by pump 130A and bladder 566 is vented to allowfor the free expansion of bladder 522.

At process block 570, series of pulse measurements, as has beendescribed before with respect to the array structures of this invention,are made but with an alternating of the transmitting transducer betweenthe left and right side of the heel 207 with the opposed transducerserving as the receiving transducer. Pairs of such measurements may beaveraged together to reduce the variation in heel measurement, however,it will also be understood that other statistical techniques may beapplied to the left-going and right-going pulses to detect, for example,abnormal situations indicated by the deviation between these pulsesbeing too great.

At decision block 572 one of three operations may be performed. First,the deviation between successive measurements at process block 570 maybe compared until the deviations drop below a predetermined amountindicating that an asymptote in the measurements has been reached.Alternatively and preferably, the successive ultrasonic measurements maybe fit to a decaying exponential or other similar curve until thedeviation between that projected curve and the actual curve drops belowa predetermined amount. Then the asymptote for the projected curve maybe used as a final value. Alternatively, attenuation and speed of soundmeasurements may be made at process block 570 and combined to reduce theasymptotic variation in the successive measurements. These combinedmeasurements may also be fit to a curve as previously described.

Referring momentarily to FIG. 34, the present inventors have recognizedthat in apparatus of this type, the measured values of sound speed 574tend to decline with time whereas measured values of broad bandultrasonic attenuation (BUA) 576 tend to rise with time. A combinationof these values reflected in a stiffness measurement 578 such as may bean empirically weighted sum of BUA and SOS converges more quickly to theasymptote value 580.

Referring again to FIG. 33, once an asymptote value for the ultrasonicmeasurements has been determined from which bone health measurements canbe made the process may conclude. Alternatively, the coupling liquid Afrom bladder 522 is withdrawn and bladder 540 is inflated with couplingliquid B having a different sound speed as shown in FIG. 31 by processblock 582. The pressure and temperature of the coupling liquid B arecontrolled to be the same as coupling liquid A. As before, with processblock 570, interleaved left to right, right to left ultrasonicmeasurements are then taken as indicated by process block 584..

Next, at decision block 586, an asymptote value is again derived, as hasbeen described, and the program proceeds to process block 588 wherewidth of the heel is calculated by the following means:

Assuming that liquid A has a sound speed of A, liquid B has a soundspeed of B, the soft tissue about the heel has a sound speed of C, andthe os calcis 216 has a sound speed of D, then the time of flightbetween the transducer arrays 300 will be described by the followingequation:

t ₁ =w ₁ /A+w ₂ /C+w ₃ /D  (4)

where, w₁ is the total distance between the transducer array 300 and theheel, w₂ is the total distance through soft tissue, and w₃ is the totaldistance through the os calcis 216. Similarly for coupling liquid B, thetime of flight will be

t ₂ =w ₁ /B+w ₂ /C+w ₃ /D  (5)

Accordingly, a difference in time of flight with coupling liquid A and Byields a value of the heel width w₁ as follows::

w₁=(t₁−t₂)/(A−B)  (6)

If the os calcis is assumed to have a substantially constant width, theamount of soft tissue may be deduced and this width of soft tissue orthe heel width itself may be used to make an empirical correction to thebone health measurement as previously described.

It will be understood from this description that the same procedure maybe adopted using coupling liquids A and B that have differentattenuations and in fact liquids having both variations in attenuationand sound speed may be used.. Further, all the bladders may bereplaceable through the use of sealing members such as o-rings toprovide a good seal on a replaceable bladder.

Oversized Bladder

Referring now to FIG. 35, the shape of the flexible membrane 626 in theforegoing examples is controlled in shape so as to provide a maximumresistance to movement of the patient member 207 across the axis 212when the member 207 is in contact with the membrane 626. In the priorart, membrane 626 a was attached about transducer 300 at a periphery 654a conforming closely to the active front surface 656 of the transducer300. During operation, the membrane 626 a extends a distance D alongaxis 212 by an amount intended to bridge the gap between the transducers300 and a patient member (not shown in FIG. 35) for different sizedpatient members while providing ample clearance for insertion andremoving of the patient member.

The present invention recognizes that superior patient immobilization isobtained by expanding the point of peripheral attachment of the membrane626 b to an outer periphery 654 b substantially greater than theultimate extension distance D and preferably greater than twice D.Generally, it is desired that the surface of the membrane 626 extendingthe distance D while retaining a generally hemispherical shape such asis believed to provide greatest resistance to cross axis motion.Recognizing that the membrane 626 may take on other than a circularperiphery, the diameter of the periphery as used herein will refer tothe diameter of a circle circumscribed within the periphery 654 b.

Referring now to FIG. 36, the advantage of the expanded periphery 654 bmay be seen by imagining the displacement of a point 658 a on themembrane 626 to a new location 658 b displaced across axis 212 such asmight be caused by motion of the patient member. Such motion wouldrequire a substantially greater stretching of the membrane 626(b) withrespect to the periphery 654 b as shown by displacement distance 660than it would require of the membrane 626(a) with respect to theperiphery 654 a as shown by displacement 662. A greater displacementfrom these points corresponds to a greater stretching of the curvedmembrane 626(b), thus a greater restoring force provided by the membrane626(b) in preventing that movement. Motion of the patient member alongaxis 212 may be controlled by controlling the pressure within themembrane 626 to provide the desired degree of immobilization.

Inflated Bladder with Fixed Bladder Support

Referring now to FIG. 37, the benefits of a closed environment for theacoustic coupling fluid 640 may also be obtained in an inflation-typesystem wherein cylindrical chambers 664 having open ends facing eachother along axis 212 with front opposed surfaces enclosed by membranes626 and rear surfaces connected by means of hydraulic tubing 666 to pumpchamber 668. The chambers 664 are fixed in separation and with respectto the transducers contained therein. The transducers may be anycombination of arrays 300 and/or individual transducers 121 as have beenpreviously described. Valves 641 communicating with the interior of thechambers 664 may allow removal of coupling fluid 640 or the bleeding outof entrapped air. An open face of the pump chamber 668 may be closed bya rolling diaphragm 670 being essentially a flexible membrane attachedat its edges to close the chamber 558 and attached at its center to apiston 672. Displacement of the piston 672 in toward the chamber 668creates a predefined reduction in volume of the chamber 668 causing,through hydraulic equalization, a distention of membranes 626 outwardtoward the patient member contained between them. The rolling diaphragm670 requires no sliding seals, as are found in conventional pistonpumps, nor creates the possibility of backwash as can occur withperistaltic-type pumps. Further, the flow rate provided to the chambers664 need not be measured in order to determine the amount of distentionof the membranes 626 as there will be a fixed and easily calculatedrelationship between movement of the piston 672 and distention of themembranes 626. This relationship is not as easily calculated with pumpsrequiring check valves or having backwash or leakage. By hermeticallyclosing the coupling fluid 640 from outside environments, the couplingfluid may be degassed and contamination may be essentially eliminated.

The transducer chambers 664, interconnecting hydraulic tubing, and 666pump chamber 668 all may be partially formed from or incorporated into athermally conductive matrix such as an aluminum block to form aneffective a single heating chamber whose temperature may be controlledby activation of resistance heating elements 164 also in thermalcommunication with the heating chamber 132 and monitored by thermistor168 as previously disclosed. This arrangement eliminates the need forseparate thermal regulation circuitry for each of these elements andprovides good uniformity of temperature of the various elements.

The chamber 558 may be in communication with a pressure sensor and thepiston 672 further controlled to monitor the pressure in the chamber 558to substantially 1 PSIG. As will be described below, this same mechanismmay be used to pre-inflate the membranes 626 prior to insertion of thepatient's foot. In this case, the pressure sensor may be used to monitorthe pressure as a function of volume of liquid in the chambers 664, andhence inflation of the membranes 626, to determine that the patient'sfoot is not in place during the inflation process. Presence of thepatient's foot would increase the pressure sensed by the pressure sensorfor a given inflation volume and thus may also be used to confirm thatthe patient's foot has been inserted.

Pre-Inflated Bladder System

Referring now to FIG. 38, an ultrasonic densitometer 710 includes ahousing 712 having an aperture 714 and its upper surface exposing areceptacle 718 sized to receive a human heel therein when the housing712 placed on the floor in front of a seated patient.

Attached to left and right walls 716 of a receptacle 718 and positionedbelow the aperture 714 are left and right bladders 720 presentingopposed convex bladder surfaces 722 formed of distended membranes. Itwill be recognized that a similar system may be used with a singlebladder in a reflection mode where one of the bladders 720 is replacedsimply by a soft material providing support for the side of the heel.Further, the bladders 720 may be used not with single transducers butwith transducer arrays or a combination of single transducers or singleelements of array transducers and arrays may be used.

Referring also to FIG. 39, prior to insertion of the patient's heel 724,the bladders 720 are pre-inflated so that surfaces 722 definetherebetween a cavity sized to be smaller than a standard human heel724. The bladders 720 are pre-inflated with a coupling liquid, typicallywater 726, to distend a sheet of silicone rubber to a nearlyhemispherical configuration as described above. While the bladders 720are pre-inflated before the heel 724 is inserted, they are initiallyinflated by a pump system such as described above and may be deflatedfor storage or shipping. The act of inflation is, of course, intrinsicto any inflated bladder.

The water is contained between each bladder surface 722 and acorresponding backer plate 728 is attached to the membrane of thebladder 720 at its circular periphery. Positioned within the cavitydefined by the backer plate 728 and the bladder surface 722 is anultrasonic transducer 730 as described above. The transducers 730 andbacker plates 728 are held by structure 721 in fixed separation andessentially fixed with respect to the foot plate 738 as will bedescribed.

The ultrasonic transducers 730 are aligned along an ultrasonicpropagation axis 732 extending therebetween and intersecting the centerof the bladder surfaces 722. A thin coating of ultrasonic coupling gel734 is placed on the outer surface of the membrane forming the bladdersurface 722 to provide lubrication and coupling for the later insertionof a heel 724.

Referring now to FIGS. 39 and 40, the heel 724 may be inserted in adownward direction 736 across (i.e., perpendicular) to the ultrasonicpropagation axis 732 so as to slide pass past the bladder surfaces 722deforming them inward, the heel 724 to abut a heel plate 738 stoppingfurther downward motion of the heel 724. The elastic nature of themembranes of bladder surfaces 722 causes the surfaces 722 to deform asthey slide along the outer surfaces of the heel 724 in a wiping actionreducing entrapment of air between the bladder surfaces 722 and the heel724. The hemispherical shape of the bladder surfaces 722 and thematerial of their construction allows them to conform to the shape ofthe heel 724 and remain substantially aligned with the ultrasonictransducers 730 during the insertion of the heel in contrast to otherbladder shapes which as a result of their longitudinal extension orconical shape may collapse or fold over with the insertion of a theheel.

Referring now to FIG. 41, the heel plate 738 may generally have a lowersurface 740 supporting the sole of the foot and back surface 742perpendicular to surface 740 supporting the back of the heel 724.Surfaces 740 and 742 may be generally perpendicular to each other with740 tipped upward toward the toe end with respect to the upper surfaceof the housing 712. The toe end of surface 740 may rest to pivot about alower support fulcrum 744 while the upper edge of back surface 742 maybe engaged by a stationary load cell 746 fixed with respect to thehousing 712.

The load cell 746 may measure downward pressure 747 of the heel 724 onthe plate 738 and backward pressure 749 of the heel 724 on the surface742 so as to insure proper seating of the heel 724 against the plate 738and proper location of the ultrasonic propagation axis 732 in thedesired region of the os calcis 748.

Referring now to FIG. 42, the downward force 747 (F1) and backward force749 (F2) may be monitored by the microprocessor 38 (described above) soas to ensure that the forces (F1 and F2) of a given measurement liewithin a desired range 750 indicating proper seating of the foot againstthe plate 738 without undue force thereon and indicating further thatthe calf of a patient's leg abutting a calf support 120 (shown in FIG.38 in phantom) is not holding the back of the heel 724 away from surface742.

In an alternate embodiment, not shown, the bladders 720 may bepre-inflated so as to touch and in this way provide an acoustic pathdirectly from one ultrasonic transducer 730 to the other without air gapor intervening heel 724. This provides a separation distance of zero.This configuration may be used to provide a pre-measurement standardpulse between ultrasonic transducers 734 as has been previouslydescribed, and/or to provide a positive indication (by measurement ofthe ultrasonic propagation) as to whether the patient's heel 724 hasbeen inserted between the bladders or not. The shape and composition ofthe bladders 720 is such as to allow the foot to slide between thebladders 720 even when there is initially no appreciable gap between thebladders 724.

It will be within the understanding of one of ordinary skill in the art,that the features of the various embodiments described herein may beinterchanged with other embodiments to effect the purposes describedherein and therefore that the inventor contemplates the construction ofcommercial devices including either combinations of features of severalembodiments disclosed herein or less than all the features of any oneembodiment. It is specifically intended that the present invention notbe specifically limited to the embodiments and illustrations containedherein, but embrace all such modified forms thereof as come within thescope of the following claims.

We claim:
 1. An ultrasonic densitometer comprising: a patient support;an ultrasonic transducer system including at least one ultrasonictransducer positioned with respect to the patient support to directultrasonic signals through a patient positioned by the patient supportand to receive the ultrasonic signals after attenuation by the patient;a coupling volume positioned between an ultrasonic transducer and thepatient, the coupling volume alternatively occupied by a first couplingmaterial having a first known sound propagation property and a secondcoupling material having a second known sound propagation propertydifferent from the first sound propagation property; and a computercommunicating with the ultrasonic transducer system to measure a firstultrasound propagation through both the patient and the first couplingmaterial and a second ultrasonic propagation through both the patientand the second coupling material, the computer executing a storedprogram to compare the first and second ultrasonic propagations toproduce an indication of bone quality of bone within the patient.
 2. Anultrasonic densitometer comprising: a patient support; an ultrasonictransducer system including at least one ultrasonic transducerpositioned with respect to the patient support to direct ultrasonicsignals through a patient positioned by the patient support and toreceive the ultrasonic signals after attenuation by the patient; acoupling volume positioned between an ultrasonic transducer and thepatient, the coupling volume alternatively occupied by a first couplingmaterial having a first known sound propagation property and a secondcoupling material having a second known sound propagation propertydifferent from the first sound propagation property; and a computercommunicating with the ultrasonic transducer system to measure a firstultrasound propagation through both the patient and the first couplingmaterial and a second ultrasonic propagation through both the patientand the second coupling material, the computer executing a storedprogram to compare the first and second ultrasonic propagations toproduce an indication of bone quality of bone within the patient;wherein the coupling volume holds a first and second overlappingflexible bladder alternately filled with liquid first and secondcoupling materials.
 3. The ultrasonic densitometer of claim 1 whereinthe first and second coupling materials have different soundattenuations.
 4. The ultrasonic densitometer of claim 1 wherein thefirst and second coupling materials have different sound propagationspeeds.
 5. An ultrasonic densitometer for measuring bone in vivocomprising: an ultrasonic transducer system including at least oneultrasonic transducer positioned with respect to the bone to directultrasonic signals through the bone and to receive the ultrasonicsignals after attenuation by the bone; a computer communicating with theultrasonic transducer system to: (i) measure a series of ultrasoundsignals through the bone as a function of time; (ii) fit the measuredseries to a predetermined asymptotic curve as a function of time; and(iii) output a measurement based on the asymptotic curve based on aportion of the asymptotic curve beyond the measured series.
 6. Theultrasonic densitometer of claim 5 wherein the transducer array systemincludes two transducers opposed about the bone and wherein theultrasonic transducer system alternates the direction of propagation ofultrasound through the bone between the two transducers.
 7. Theultrasonic densitometer of claim 5 wherein the measure of the series ofultrasound signals is a measure of a speed of ultrasonic propagation. 8.The ultrasonic densitometer of claim 5 wherein the measure of the seriesof ultrasound signals is a measure of ultrasonic attenuation.
 9. Theultrasonic densitometer of claim 5 wherein the measure of the series ofultrasound signals is a combination of measures of speed of ultrasonicpropagation and ultrasonic attenuation.